Rainbow
Valkyrie
WonderShot
Crisis Arm
Masamune 2
Bronze Fist
DoomSickleSo, are you the type of person that wants to get the most out of your characters in a RPG? Whether you max out stats and levels or just want to beat the game, it's safe to say that under normal conditions players want their characters to be effective, and use whatever means they can to make their characters stronger.
In Chrono Trigger, there are three ways to customize your party's general effectiveness: levels, equipment, and stat items ("tabs"). Levels are gained through experience points from battles, equipment is bought and found all over the game, and tabs are special items that increase a single stat by one point. All three factors are important in strengthening your characters in the long run.
One more note: I say "at the end of the game" or "endgame" a whole lot in this guide. If you aren't sure how far in the game you are, check the subtitle of your saved game. If it says "The Fated Hour" or "The Final Battle" then these endgame quests are all available to you. If not, then you have some storyline to clear before you can get item hunting for that item "at the end of the game." However, when I say "when you are able to open black chests," that refers to a plot event somewhat earlier in the game. You'll know it when you see it.
My last note, I promise: Charm (Ayla's fifth tech) is a very important tech for obtaining strong equipment and tabs. Charm is essentially the game's item-stealing skill. If Ayla's level is too low to Charm effectively and consistently, then try using the dual tech Twin Charm (Ayla and Marle) for better odds.
How high do your levels need to be to finish the game? Not terribly high. In Chrono Trigger, the maximum level is 99, displayed as ** in your status screen. Maxing out levels is not required to beat the game's most difficult enemies, and is only for players that are willing to put in several more hours of game time. With each level, each character gains in stats a predetermined amount (sometimes zero) - the only stat that never changes from levels up is Speed. Many attacks and techs have damage formulas that take character level into account, so as a general rule your party will always grow stronger with additional levels. Generally most players are able to beat the late-game bosses at levels in the neighborhood of 40-50. Depending on how much time you spend leveling up and what equipment you have on, a suitable level for beating the game could go far above or below that estimate. For those interested in leveling up large amounts, the best areas to level are the game's final sets of dungeons available to you - anywhere in the Black Omen, the conveyor belt in the Geno Dome, and anywhere in the Giant's Claw are good choices. Before that, the Mountain of Woe is a good way to get some experience and TP too.
I feel that this is a bit of a silly question. (And you're a silly person for asking it, reader. -Ed) All seven playable characters in Chrono Trigger are very good. All of them have good techs, all of them are capable of both offense and support roles, and any of them can make an effective team. For most of the game your party has varying amounts of flexibility, but by the end game you can fully customize your team's lineup. When that chance comes, just pick your favorite characters and roll with it. My favorite party at the end of the game is Crono, Robo, and Ayla, but I certainly mix and match depending on the situation and my mood.
Note: This section switches answers based on the version you're playing; make sure if you only care about the NDS version that you select it from our navigation!
RainbowValkyrieWonderShotCrisis ArmMasamune 2Bronze FistDoomSickleWith regards to armor and helmets, this is a little more difficult to answer than the weapon question. I'll profile a number of very good endgame armors, and you, the player, can decide how to allocate them.
PrismDress and 
Prism Helm: after completing the Rainbow Shell quest, you have a decision to make. You must choose between three Prism Helms or one PrismDress. Prism Helm are the best helms in the game for all characters other than Magus; the PrismDress is the best armor for Lucca, Marle, and Ayla. Choose carefully. You can also Charm both a PrismDress and a Prism Helm from Zeal 2 on the Black Omen (her head for the dress, her left hand for the helm). The PrismDress has excellent defense and reduces all magic damage to its wearer by 1/3 with an inherent MagicWall status. The Prism Helm has very high defense and also protects against bad status effects, making it the best helm in the game for all characters other than Magus.
Moon Armor: you obtain the Moon Armor in the same way you obtain the Valkyrie bow—follow the guide for using Marle's Pendant. The Moon Armor is the highest-defense armor available for Crono, Robo, and Frog (but Magus can wear it too). It also adds 10 to Magic Defense.
ZodiacCape: the ZodiacCape is the second-strongest feminine armor (only Marle, Lucca, and Ayla can equip it), and it adds 10 to Magic Defense in addition to possessing high defense. There is one ZodiacCape in a chest in the Giant's Claw, and another in the Black Omen.
Nova Armor: there are several ways of obtaining Nova Armor—it is found in chests in the Black Omen and in the Northern Ruins (600 AD), and it can be Charmed from Goons in the Black Omen. Nova Armor is masculine armor, so it is only available for Crono, Robo, Frog, and Magus. Nova Armor has less defense than Moon Armor, but it does prevent adverse status effects just like a Prism Helm or Vigil Hat.
Taban Suit: at certain points in the game, you can visit Lucca's father Taban to acquire specialized equipment for Lucca. If you visit Taban at the end of the game (he's in Lucca's House in 1000 AD) he'll give you the Taban Suit, which grants +3 to Speed as well as increases her resistance to fire attacks. Since Lucca has the worst Speed in the game the speed boost is welcome, and against certain bosses (Son of Sun and Rust Tyrano, for example) fire defense is great to have. The defense lags behind those of other armors found at the end of the game, but it's a viable option for the final few areas. If you visit Taban and he gives you a Taban Vest or Taban Helm instead, or doesn't have anything at all, then don't worry. You're either too early in the game to get the Taban Suit, or you need to visit him again since he can only give you one item at a time. It's worth noting that the Taban Suit, is not carried over into a New Game+ (nor are other Taban items).
Gloom Cape and Gloom Helm: these are the strongest armor and helm for Magus. They are both located in the secret passage in Ozzie's Fort in 600 AD, the same place as the DoomSickle. Both have very strong defense, and the Gloom Helm protects against negative status effects.
Mail With All the Colors of the Wind: when you gain the ability to open black chests, there are a few chests in 600 AD that are also present in 1000 AD. Follow the instructions and in four of these chests you find the Black, Blue, Red, and White Vests. If powered up, these vests become Mail and gain a very large increase in defense as well as complete protection from one element. These four chests (in both eras) are located in Truce Inn (Blue Mail), Porre Elder's House (Black Mail and White Mail), and one of the stairwells in Guardia Castle (Red Mail). These armors aren't as good as the top-level armors, but are excellent when you get them and reasonably good for the final boss. If you want a second copy of each Mail, you can Charm one from four endgame bosses: Son of Sun, MotherBrain, Yakra XIII, and Rust Tyrano.
Ruby Armor: Ruby Armor may not be as good as the armor listed above, but it's very, very easy to obtain and you can have as many as you want, so I decided to put it down. After the major events of 12,000 BC visit the trader in Ioka Village in 65,000,000 BC and give him 10 Petal, 10 Fang, 10 Horn, and 10 Feather for one of these. Go to the Hunting Grounds northeast of Ioka if you need the materials. The Ruby Armor has great defense and is very strong against fire attacks, plus you can get as many as you want. Great before you get the top-quality stuff.
Vigil Hat: the Vigil Hat is found in several areas, including the Black Omen. However, it is most easily obtained by completing the Fiona's Villa quest near the end of the game in 600 AD; after completing the quest, visit Fiona's Shrine in 1000 AD and you can purchase Vigil Hats from one of the shrine sisters present. The Vigil Hat protects against bad status effects and possesses very high defense, making it a good endgame selection for any character lacking a Prism Helm, Nova Armor, or Gloom Helm.
Haste Helm: the Haste Helm has lower defense than the other headgear in this section, but it automatically gives your character the Haste status. With Haste running, your character's speed increases significantly and as a result some players prefer the Haste Helm to other late-game helms available. There is one Haste Helm in a chest on the Black Omen, and you can Charm a second one from a Lavos Spawn in the Black Omen.
Safe Helm: when you gain the ability to open black chests, visit the glowing pyramid northeast of Medina Village in 1000 AD. There, you'll break another magic seal and must choose between two special items: the Safe Helm (on the left) and the Swallow blade (on the right). The Swallow is a good weapon for Crono, but not as good as the Rainbow. The Safe Helm is a little more useful, possessing very high defense and further increasing survivability with a permanent Protect status. Protect reduces all physical damage to its wearer by 1/3.
Special Note: when I make the choice between the Safe Helm and the Swallow, I always choose the Safe Helm unless I'm trying to fill out a weapon list. The Swallow is useful for the middle of the game, but it will eventually get outclassed by the Rainbow. The Safe Helm, on the other hand, is a worthy helm for use in the endgame.
The choice between three Prism Helms and one PrismDress is trickier. If you use at least two of the three female characters often, go for the PrismDress; it's the best armor available in the game, for any character (but only the girls can use it). The Prism Helm is a great piece of equipment, but it has better alternatives than the PrismDress does. Going from a Prism Helm to a Vigil Hat is a loss of only 4 Defense, while going from a PrismDress to a ZodiacCape is a loss of 19 Defense. If you only use one female character and are willing to put the other two on the sidelines for the final gauntlet of bosses, then grab those Prism Helm. I like using every character, so I usually go for that PrismDress when I make the choice, and I always remember to Charm a Prism Helm and a second PrismDress from Zeal 2 in the Black Omen.
This is a question that has no right answer. Accessories are items that grant the character equipping it special abilities that often cannot be obtained any other way. I'll give info on a few of my favorite accessories, but I strongly encourage you to inspect any accessory you find and use what you like best.
PrismSpecs and Sun Shades: you can only get one PrismSpecs in a single playthrough of the game, but if I could I would give them to everyone in my party with the possible exception of Marle. PrismSpecs (and its weaker cousin the Sun Shades) increase your damage. All of it. Your attacks, techs, and whatever else will do about 50% more damage (25% for Sun Shades). That's a lot when you think of how much damage the game's heavy hitters are capable of. You get your PrismSpecs if you've completed both the Sun Stone quest and Rainbow Shell quest near the end of the game. Just speak to Melchior in the basement of Guardia Castle, and he'll hand you PrismSpecs along with the mighty Rainbow. You obtain the Sun Shades by completing only the Sun Stone quest at the endgame, but who cares? PrismSpecs!
Gold Stud: there's one of these behind the sealed door in the Trann Dome in 2300 AD, but you can also Charm them from the pink scouter enemies in the Black Omen, so if you really want to you can have one for each character in your party. Gold Studs are great; they divide a character's MP consumption by 4, making expensive techs like 
Luminaire, Flare, Triple Kick, and DarkMatter cost a mere 5 MP. If you like to use techs often and prefer not to spend your days huffing Ethers, then Gold Studs are welcome additions to your inventory. If you're near the game's end, you aren't sure what to give a character, and PrismSpecs aren't available, just steal a Gold Stud for them. Works like a charm.
FrenzyBand: this is a good accessory for Crono or Ayla - when equipped with this, a character will automatically counterattack any physical damage with a regular attack 80% of the time. It's great for strong physical attackers when you're a damage-minded player. Unfortunately, there's only one FrenzyBand, located in a chest in the Giant's Claw.
GreenDream: you obtain this nifty item automatically after the Fiona's Villa sidequest at the end of the game. The GreenDream works like a "Life 3" spell from some other Square and Square-Enix games. If your character with the GreenDream dies in battle, s/he automatically revives immediately after death, without requiring the use of a Revive item or a Life tech. This only works once per battle. It's a great accessory for Marle, since she'll be the number one healer in any party she's in and she's the only character able to cast the very useful Life 2 spell. You don't want Marle dead for long if she's in your party.
Dash Ring: At the end of the game I don't like using weapons that only increase stats a small amount, except for the Dash Ring. +3 to Speed is a solid boost to any slower character, especially for launching an attack on the first turn. I like giving one to Robo for most of the game's second half. You can find one in a black chest in Heckran Cave in 1000 AD.
That's completely up to you. Once you finish the sequence on Death Peak in 2300 AD and reach the game's penultimate chapter (The Fated Hour), most of the equipment listed above is available to you. Here's a short recommended order to do sidequests, based on my personal preferences and an estimated difficulty curve - the easiest and most beneficial quests first, and the more difficult and less beneficial quests later. For specific details on each quest, refer to the appropriate section of CoN's guide. Remember, this is only during The Fated Hour or later and this is only my preferences. If you like using Magus, then please go ahead and do #7 earlier! It's all up to you.
Taban Suit.Red Mail and Ruby Armor to your characters for Fire protection against Son of Sun (if you don't have any Ruby Armor, go and trade for a few). This gets you the WonderShot and the Sun Shades. Masamune 2, the Valkyrie, a Nova Armor, and a Moon Armor, plus the powerful Shiva Edge weapon for Crono.ZodiacCape, your choice of three Prism Helms or one PrismDress, the PrismSpecs, and the Rainbow. Remember, those last two only come if you've already completed the Sun Stone quest. Crisis Arm for Robo, a permanent increase to a few of Robo's stats, and more.Gloom Helm, Gloom Cape, and DoomSickle, plus some other good treasure.Vigil Hats.PrismDress and Prism Helm Charmed from Zeal 2 and the opportunity to Charm Gold Studs and Nova Armors. Going through the Omen multiple times, backwards through time, will get you even more stuff.That's my preferred order of the endgame sidequests. If you're really, really determined to get a save file with the best equipment equipped to each character, you'll need to run through the game at least three times using a New Game + due to the scarcity of Prism Helms, PrismDresses, and Moon Armors. Happy item-hunting!
Tabs are very important little items that increase their three namesake statistics by one point for each one you use. I strongly recommend you check out CoN's list to make sure you don't miss any. But anyhow, since these tabs are for the most part limited in number and have unlimited demand, how should you use them?
Power Tabs increase Power, naturally, which in turn increases the damage of regular attacks (for all characters other than Lucca and Marle) and physical techs. The maximum Power any character can have is 99, represented by ** on the status screen. Magic Tabs increase Magic; Magic increases the damage of all magic techs, the effectiveness of all healing spells, and the damage of a few elemental techs such as Lucca's bombs and Robo's Laser Spin. Just like power, Magic maxes out at 99/**. Speed determines how quickly your characters act in battle. Speed Tabs are precious, because Speed does not increase by gaining levels. The only way to increase one's speed permanently is to use Tabs - the exception to this rule is Robo, who can permanently increase his speed by 3 (moving from a Speed of 6 to a Speed of 9) in the Geno Dome sidequest. Speed hits its double-star value at level 16.
So now you know how Tabs work, let's get to the part about using them. First, if you use Tabs to increase stats, it does not interfere with stats gained from levels - no matter when you use any given Tab, you'll always gain the same amount of stats at each level. Thus, you should be able to use Tabs at any time without worrying about wasting them or having less-effective level-up. But before using a Tab, you should ask yourself: "how much do I want to level up in this game?" and "am I going to do a New Game +?" First let me explain - if you're going to level up all the way to level **, then most of your characters will max their Power and Magic stats out anyway. Why give Lucca a Magic Tab if she can max out her Magic by herself with a few more levels? If you think you'll be doing a LOT of leveling-up in Chrono Trigger, then hold off on using your Power Tabs and Magic Tabs right away. On the flipside, if you are going through the game quickly and don't want to do much leveling up, then go ahead and use Tabs as soon as you get them. You won't be penalized for early Tabs use.
And about the New Game +, well, that kind of throws it all off-balance. NG+ allows you to start a new game with nearly all of your stats, levels, techs, inventory, and equipment from a file that's already beaten the game. If you're planning on eventually running through a New Game +, then you may eventually have double the usual number of each Tab, allowing you to use them that much more freely.
But who should get which? It depends on your playstyle, but there is one rule: Marle and Lucca should never get Power Tabs. Why? Because those two will never use those stats, for any reason. Those stats are completely vestigial. Marle and Lucca's physical attacks deal damage using the Hit stat instead of the Power stat. If you want to max them out for the thrill of it, then go ahead, but it won't do you any good. Other than that, you can pretty much use your own judgement, but I say this: use Tabs on your favorite characters. You'll be using those characters the most anyway, and you'll benefit the most from it. So that's the bottom line on using Tabs: save them for your favorite characters to use, and pay attention to CoN's guide. Many hard-to-find Tabs (especially Speed Tabs) need to be Charmed, as well.
So that's all. Now you know what leveling up does for your characters, what's the best equipment in the game and how to get it, and what you need to think about before you use all of your tabs. Now go and conquer Chrono Trigger!