Sonic & Knuckles Music:
Lava Reef Zone
Death Egg Zone – A challenging zone that mind-fucks with you the whole way through. Sonic Team went all out in the level design here.
Doomsday Zone – The first time you had Super Sonic exclusive boss. Unforgettable.
Also, forgot the Sonic 3 credits music.
There is long-standing supposition going around the Sonic fandom that Michael Jackson helped create the music for Sonic 3, but Sonic Team have neither confirmed nor denied it. But I’m inclined to believe it, given how much some tracks resemble Jacko’s musical style (the credits theme matches “Stranger in Moscow” beat for beat).