I like a bunch of the Mega Drive stuff, but I feel that sometimes it's just being too abstract and noisy. The Sonic Advance series has art that reads way better, and I feel the general shape of things is also much nicer and clearer.
But what the hell do I know about pixel art, right?
[EDIT] I think Leaf Forest (Storm was the Sonic Rush zone) looks awesome. It's has a classic Sonic look with some abstract shapes, but stays perfectly clear and readable. This goes for a lot of the zones in the Sonic Advance games. It's a great balance between crazy, abstract shapes and good readability for the player.
Also, how is Leaf Forest any different from something like Green Hill Zone? It being a cliché Sonic level (like most of the Advance stages, I'll give you that) has nothing to do with the art not being top notch.