Gotta be one of the best sequels out there. I think the run of the first 5 zones in Sonic 2 (Emerald Hill - Hill Top) might be consistently the strongest and most fun set of levels in the franchise.

The games let down a BIT by its last couple of zones though. Mystic Cave is... fine, but Oil Ocean is super ugly and boring and then Metropolis is obnoxious at times to get through.