Too hard to choose and I think it would have been made easier if you did Mario platformers and a separate poll for spin-offs, so I'm going to limit myself to the former to make it easier on myself :)
2D - Super Mario World, simply awesome, still. Yoshi's Island a close second and honourable mentions to pretty much every other 2D Mario.
3D - The Galaxies, nuff said. 3D World only a lick behind with its awesome multi-player. Mario 64 gets an honourable mention. As revolutionary as it was and as awesome as it was first stepping onto Bob-omb Battlefield for the first time (i don't think that feeling will ever be matched), I feel it lacks the gameplay density of the newer games.







