Yoshi's Island and Wario Land 2 (if that counts) are probably my favourites. Super Mario Bros. 2 (Doki Doki Panic, not Lost Levels) may come in third. Super Mario World has the best movement/physics engine out of any Mario platformer for me, but the badly balanced items and rather lackluster level design harm it a lot. The Game Boy Adance remake only made it even more cheesy instead of fixing anything.
The 3D games are the most fun to watch someone else play, but I don't like playing them myself.
The 3D games are the most fun to watch someone else play, but I don't like playing them myself.