The Biolizard Fight in Sonic Adventure 2.
This song really fits the mood of the fight and the recent events of the story involving Shadow.
Side note, I don't understand why people have such a hard time with this boss. I have this game on Dreamcast, GameCube, and PSN and i find it very easy. I hear people complain about falling in to the water but if you watch the floor you notice the tiles of the outter rim are a dark color so its easy to know when to jump... But whatever...