It could be the whole "childhood game" thing clouding my judgement and giving me a bias towards this game, but I think Buster's Hidden Treasure is an underrated classic for the Sega Genesis and probably the closest us Sega kids got to a Super Mario World on the system. Yes, you do have to deal with the unwieldy password system to save your progress, but exploring the levels and finding the secret levels was, as they said in the 90's, "awesome as hell", and the soundtrack is incredible.