Sakaki from .Hack G.U.
Mostly because the fucker just wouldn't quit. Ruin his planes once? He comes back using the game's form of a God to fight you. Ruin that plan? He comes back once more as an inhuman monster. and the entire time he just sound even crazier and more malicious than the last time you stomped him out.
I think a big part of it was the voice actor. Top notch, he really got the emotion down in his part as Sakaki.