This is a tough one for me. I like it when Ansem yells out "submit!," but I also like it when Sephiroth says "Sin's harvest" before instantly killing you (unless you have Second Chance equipped, or interfere his attack).
However, I'm choosing Sephiroth. You see how insane he really is when you defeat him. Sephy defeat spoilerAs he's being "returned to the planet," I noticed he was slightly smiling, as if he were glad to be defeated.