I'm gonna be the guy that "goes there" and pick Sky Deck, because it was a 3D level with ladders, monkey bars, anti-grav, the swinging crane gimmick, falling floors and general awesomeness. It's the most fun I've ever had in a 3D Sonic game, about the same as when I first played Sonic 3. The entire game, really, was a memorable experience that I can still have fun with today.
But oh yea. I went there.