These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
Game Breaker: Whitebeard in the first game. He seems to have a counter for every situation, such as using Haki to stun a boss character who moves around a lot, his attacks all do a crapload of damage and he has a killer range that could put Usopp to shame. Not to mention his level-2 Special can wipe an entire territory of Mooks. Sure, he's slow as all hell, but that doesn't keep him from annihilating everything in sight. Still, Whitebeard is the World's Strongest ManIn-Universe, so it's probably justified.
In the second game, this honor goes to multiple characters;
For starters, there's every single Logia user in the game(except Smoker), particularly the Admirals and Eneru; all of them have high-damaging level-2 Specials that hit in a huge radius(especially Aokiji/Kuzan's Ice Star, and Eneru's Raigou/Kingdom Come), along with easily-spammable moves that also deal big damage from afar or in a huge radius(SSSST/Ice Age for Aokiji/Kuzan, SSST AND SSSST combo for Akainu, SST AND TS AND TTS for Kizaru, and Maximum Voltage SSST/Julungul for Eneru; Eneru also has SSSST/Mamaragan for Elite Mooks or stronger characters, if you can force them into a corner). Like Whitebeard, this could be justified by the fact that these characters are just that damn powerful In-Universe.
Not to leave out Crocodile, though; his SSST(Sables) sucks in nearby Mooks as it moves forward, dealing multiple hits of damage to them and bunching them together for any kind of nastiness you want to do to them. For Elite Mooks or stronger, Crocodile has his SSSST(Sables Pesado), which moves forward like Sables but doesn't suck in enemies, instead flooring any enemy it comes into contact with, dealing BIG damage. Not to mention, of course, Crocodile's Level-2 Special(Ground Death), which hits in a large radius.
Then there's Bartholomew Kuma; just like the above Logia users, he has an easily-spammable move, though Kuma's can hit from a truly ridiculously long range and pretty much instant-kills lesser Mooks AND can even chunk Elite Mooks(SSST), he has a single-target hard hitting move for Elite Mooks and other characters that can leave them set up for more attacks or even the same combo again(SSSST), his Style Action is Technical-type so all enemies will be slowed down during use, AND he has a Godlike level-2 Special(Ursus Shock, of course). Did I also mention that his level-1 Special heals him(albeit not by a whole lot), and the SSST spam will effortlessly build up lots of Meter so you can throw his Specials with little-to-no consequence?
Let's not forget Dracule "Hawkeye" Mihawk, either; his Gameplay gimmick is that after he does a full SSSSS combo, he builds up a Charge of power and can do the SSSSS combo again but more damaging. He can build up two charges of Power(3 repetitions of SSSSS), and the extra Power also applies to the T Finishers on said S Combo; one of his best Mook-clearing normal combos is [SSSSS x2]SSSST, with the SSST for single targets. He can also charge up lots of Meter for Special attacks as a result, and may God have mercy on his enemies once he enters Style Mode(where one of his best Combo finishers is SST, as it sends projectiles EVERYWHERE in front of him). Lastly, his Unique Action is a single quick slash... That can be canceled into any of his normal Combos and immediately grants a Combo Charge regardless of whether it hits an enemy or not, so you can get to the fully charged Finishers quicker. World's Greatest Swordsman, indeed...
A lesser example would be Teach; yes, he's not as overpowered as any of the other Logias in the game in terms of spammable combos(except Smoker), but his Specials suck in enemies like crazy. Add on the Warrior's Wisdom skill, and you can get nearly 100 Kills easy with just his level-1 Special (his level-2 special is even better in this regard).
There's also Double-Ability Teach(separate from normal Teach, who only has the Dark-Dark powers; Double-Ability Teach has the Dark-Dark and Whitebeard's Tremor-Tremor powers). All that needs to be said about how stupidly strong this guy is, is that one of his Specials is Heaven & Earth Shaker, Whitebeard's above-mentioned Special that can clear out a massive area of Mooks... Except for Double-Ability Teach, this Special is his level-1. You read that right; for the same cost as Whitebeard's game-breakingly strong level-2 Special, Double-Ability Teach can do the same Special twice.
Moral Event Horizon: In Pirate Warriors 2: Teach and Moria use the Fog Dial to make Whitebeard and his crew kill each other, resulting in Whitebeard getting hit by Akainu and the Straw Hats and the Whitebeard Pirates' allies tiring out a berserk Whitebeard. After Whitebeard is weakened enough, Moria kills him by stealing his shadow and Teach twists the knife by stealing his Tremor-Tremor Powers.
That One Achievement: A truly patience-trying one in the form of "We're Running Away! We Stole Some Gold!" The player is required to activate every coin skill in the game. It sounds simple, but players of the game will know just how painful coin collection is after a certain point, since coin collection will become a huge Luck-Based Mission after a while, no matter which stages and characters are used. Worse, one of these coin sets consists of coins that only be gotten by playing online with someone in your friends list. The game doesn't exactly have a huge user base as is, and those in your friends list may very well not have the game. Hope you get it before the servers shut down!
That One Boss: AdmiralKizaru, at least in PW1. His attacks are incredibly fast, he can become invincible, and teleports all over the place. You're very unlikely to keep your Beat score against him, and he's so damned smug.