Sky Dragon Slash from Grandia II and the Spectre Assault Rifle MK.X from Mass Effect both come to mind here...
Sky Dragon Slash for being a battle-wide attack that deals 3-4000 damage, charges fairly quickly and only costs 99 MP (which can be modified to 30 MP). Most enemies in Grandia II by the time you get Sky Dragon Slash have around 2-3000 HP, so it one hit kills nearly everything - hell, the final boss goes down in around 4-5 of these things, and he has the most health out of anything in game (a rare first for RPGs I've played...)
The Spectre Assault Rifle MK.X is overpowered for many reasons; 1: even unmodified it can unload a ridiculous amount of shots - modified, it's a 'Never stop firing' weapon. 2: It does ridiculous amounts of damage per bullet, which makes it's fast firing rate and 99.99% accuracy (with no recoil or accuracy loss while firing). 3: It only costs 1,000,000 credits - now, this might seem like a lot to you, but if you've played Mass Effect 1 you'll notice how ridiculously easy it is to get that much money (the MK.VII is only 300,000, and is almost as powerful). So it never stops firing, does massive damage per hit, has flawless accuracy and is really easy to get ahold of. Could only be more overpowered if it fired exploding bullets...
Also the Pulse Rifle from the Dead Space games. 175 rounds in the clip, 9 damage per shot and a stun for every round that lands... if you've got a decent aim, it's pretty much just a giant 'Fuck you' to every necromorph you come across. It's the gun that doesn't even bother with limb shots to kill shit - it just ruins everythings day with stunlocks and massive damage.
I wonder though, can pokemon count? 'Cause Black/White introduced Chandelure, a Ghost/Fire type (hits everything neutrally) that can learn Energy Ball (hits weaknesses) and has decent defenses and Mewtwo's Special attack. It can be obtained pretty early in game, and every time I use it... well, I never use anything else, as all the enemies are dead before I can switch out.