For me, it would have to be Kefka Palazzo from Final Fantasy VI.
This madman just revels in death and destruction. He loves to get his hands dirty, and make quips while he's at it.
I won't say too much since I don't want to spoil the game except I'll hint at that he succeeds where Sephiroth fails.
Quotes
“Read my lips - mercy is for wimps! There's a reason "oppose" rhymes with "dispose"...If they get in your way, kill them!â€
“Hee, hee! Nothing can beat the sweet music of hundreds of voices screaming in unison!â€
“Life... dreams... hope... Where do they come from? And where do they go...? Such meaningless things... I'll destroy them all!â€
His maniacal theme
His epic 4-part magnum opus of a final boss theme
I'll end this will a perfect cosplay of him.