For regular attacks, Demi-Fiend's normal attack in SMT: Nocturne. He just slugs the bastards. It's great. "FUCK YOU, THOR!" Pow! Right in the nutsack (Thor is tall). Nothing like taking on demons from myth and legend and beating them to death with an untrained youth throwing haymakers.
As for special attacks, probably Beautiful Three from Legend of Mana. That shit was stylish. I also loved Raging Pain. This is a hard one, though; Zell's limit break in FF8 was also extremely fun. Makoto's Seichusen Godanzuki and all of Gen's Ultras from SSF4 are awesome, too, as is Hakumen's instant kill in BlazBlue.
I think for now, I'm gonna go with Vampire Savior's Jedah, where he cuts his wrists, snatches you with a monstrous hand of blood, then takes you into an alternative dimension apparently made of blood to use what seems to be magic to beat the shit out of you while he giggles maniacally.