Probably the Sephiroth fight from Kingdom Hearts 1.
God damn, that's probably the BALLS HARDEST boss fight I had every undertaken.
If you've never had the "pleasure" of fighting him, let me break it down:
He has more health than any other enemy in the entire game. He has SO much health, in fact, that when you first start fighting him, his health bar doesn't even deplete. He actually has more health than the health system is designed to handle, so you have to beat 1 or 2 health bars out of him before he will even REGISTER damage.
His sword is frikin HUGE (seven feet long), and he can hit you from pretty much halfway across the screen... and that's BEFORE he starts randomly teleporting directly behind you to give you a steel suppository.
And, of course, there's the fact that he has his "Sin Heartless Angel" attack, which will Auto-Kill you if you don't interrupt it. And he likes to teleport to a random location on the battlefield right before he does it, breaking your target lock.
He's a tough freaking bastard. I was on the edge of my seat when I started trying to beat him for real (the first time I fought him, I was so shocked he was the secret boss that he killed me in 3 seconds, before I could react). Sweat, shaking, the whole nine yards. Now THAT'S intense.
Fucking hell.