Either the Doomsday Zone theme from Sonic 3, or this remix of the final boss music from Sonic & Knuckles:
http://www.youtube.com/watch?v=ycSJZS5VJIo
http://www.youtube.com/watch?v=B5PzRWEeKiU
http://www.youtube.com/watch?v=ycSJZS5VJIo
http://www.youtube.com/watch?v=B5PzRWEeKiU