For me, it was probably beating Sephiroth on Hard difficulty on the original Kingdom Hearts.
I remember, during my first playthrough, I went to Olympus Colliseum, saw the Gold Match, and thought, "Oh, I wonder what kind of heartless this is". Then Sephiroth appeared. I was so shocked, by the time I realized that the fight had started, I was already dead.
Fast forward to my second, hard mode playthough, and after a ton of grinding for EXP, grinding for sythesis materials, and just generally getting my ass kicked in an attempt to master his attack patterns, I finally beat him. I felt like taking my PS2 and smashing it, not because I was angry, but because I was so PUMPED after that win.
I thought beating him in Kingdom Hearts 2 would be just as satisfying... but I felt that he was way easier this time around.