In Final Fantasy 10 Lady Yunalesca could use an attack called Mega-Death. It instantly killed your whole party unless members were undead or you had prepared with a Death-Ward armor. Yes, she had a previous attack that inflicted the zombie effect, but if you didn't know you were probably curing that effect as she also liked to attack with heals on characters with the undead effect. She started the transformation into her third form with it and could use it any time during the fight. I'm pretty sure we all beat her, but if you died, it was because of Mega-Death. It really is an overpowered attack.
Any attack in a game that allows you to continuously stun-lock or be stun-locked.
Sniper Rifles tend to be pretty OP. Especially in game modes that don't let people see where you were. Gotta love killing people in areas that they think they're safe, though.
I'm going to call the Space Troopers in Star Wars: Battlefront 2 as extremely over-powered. But awesome. They come equipped with a blaster rifle, rocket launcher, and sticky grenades. While you can only use hem in space battles, and they are pretty useless on your ship, if you can get over to the enemy ship, with all the healing and ammo restoring droids, you are pretty much unstoppable. And can easily destroy most of the enemy ship from the inside. If you could use them in other battles, I would never use another class.