SA2B is a strong contender for a stage based on the music, but I'd honestly prefer a 2-D Sonic game level, because they would all, for the most part, work out so well. I'm looking at Marble Zone from the original game, and it could have so much potential. From Sonic 2, I'd like either Aquatic Ruin, or much more likely, Chemical Plant Zone. However, I'd be content with either one. From Sonic 3, there are tons of possibilities/probabilties, but none really appeal to me instantly. Sonic & Knuckles more than make up for this however, possibilities everywhere you looked... Mushroom hill would work well, Flying Battery would be incredible, Sandopolis with the ghosts and rising sand would be really cool if done correctly, the boss battle from Lava Reef, the room where you fight knuckles from hidden palace, the clouds in Sky Sanctuary, and all the cool architecture, and finally, what I consider the best choice easily...
The Death Egg Zone. This level could have conveyer belts, one above you and one below you, and periodically gravity could change. This would mean there were no ledges and the only way to KO someone would be to knock them off of the side, but it would be seriously cool.
To be fair, SA2 also has some really good possibilities, including the Pyramid Areas, Pumpkin hill/Sky Rail (You know it would be cool), City Escape (The massive truck has a stage hazard, maybe?) or somewhere on the island. I also like the suggestion of Green Forest, which I find highly likely.