It depends on the game...

KH1 - Strike Raid, this attack does way more damage than it should and its combo effect can decimate bosses, especially when you take advantage of things like Sephiroth's casting animations. Its long range keeps Sora safe from harm as a bonus so you're not as likely to blow through MP as quickly.

KH:CoM - Full Cloud Summon for Sora, King Mickey Summon and Dark Mode for Riku. Cloud's Omnislash is just unfair and the computer A.I. is not as quick to block it with a 0 card as it would like. I know too many people who just beat this game by building Cloud decks. King Mickey is pretty cheap but necessary for poor Riku since he has no other way to heal, but King Mickey is also like a Holy version of the Phoenix summon, restoring vast amounts of health for Riku and doing massive damage to everything else on the screen. Since he is his only summon, its not difficult to collect the cards and have a good stockpile for boss fights. Dark Mode is also overpowered as all hell since it allows Riku to use souped up versions of Firaga and the Cloud Summon, not to mention his regular combos are just mean.... Seriously the whole strategy for Riku is to just get him into Dark Mode as quickly as possible.

KH2 - Yeah, I'm going to concur with the MP system and all the special sub-abilities like Berserk that make the system unfair. Sephiroth is a joke when you can combo Trinity/Berserk/Heal and his only real means of getting out of your near infinite combo is to teleport in the rare moments their is a bit of lag with you pushing buttons or when you need to heal. The Form system is also an overpowered mechanic that just allows you to decimate enemies and bosses even with Heartless Mode. Master and Ultimate Mode are especially overpowered. There is a damn good reason why you can't use them for Xemnas' final form.

358/2 - It is actually a fairly balanced game for the most part, probably Limit attacks, especially Roxas' first one which is a mini-Renzoken followed by a few super powerful X-shaped holy attacks that can hit groups. Really the more unfair aspects of the game come from playing as characters like Saix, Riku, King Mickey, and Dual-wielding Roxas in the Mission mode. The stats and combos these guys have are insane, probably why Mission Mode has a much beefier difficulty mode.

Birth By Sleep - Really depends on the character, but overall, Drives are pretty damn overpowered, especially when you start unlocking second and third tier exclusive Modes. Hell, even basic ones are devastating when used under the right circumstances. For Terra, his Meteor combo finisher is overpowered, I can hit whole groups of enemies in a huge targeting area, and I am invincible the whole time and even if you had a naturally high magic or physical defense, you have no defense against this attack. Time Splicer for Ventus and Aqua, as well as Dodge Roll and Cartwheel that give near absolute immunity to damage and is the crutch that allows them to take down the games optional bosses with ease as opposed to poor Terra's whose dash move give no immunity. The Shotclocks are also pretty devastating to everything except the small handful of bosses that have some way around it. Salvation is pretty nasty for Ventus, basically his own version of the King Mickey Summon in CoM.