Super Mario World.

I haven't played SMB3 though.

Torn between Mario 64 and SMB3.

I think they've fully explored the potential of Mario 64 sequels already; I want to see a return to 2D as the main* series.

*That they care enough about to use orchestral music and not recycle graphics and sounds from NSMB DS for.

Super Mario Bros. 3 and the Donkey Kong Country games are my favourite platformers.

Super Mario Galaxy???

Honestly, not big on platformers, especially 3D ones, but I've loved most of the Kirby ones, followed by NSMB DS/Wii and the classics (Mario All-Stars)

2D Platformer: Super Mario World

3D Platformer: Super Mario Galaxy 2

Both are stunning games.

I just played again Super Mario  World. It's shorter than I remember, I finished it 100% in 4 hours, and it was very easy too, with the exception of Larry's Castle and some Special levels. Despite that, I don't know why but I enjoyed it more and think it's a better game than NSMB Wii o_O

Anyway my vote goes for Donkey Kong Country 2

Hm. Probably Super Mario 64.  Super Mario Sunshine is close behind it.

Looking specifically at 2D platformers, I'd probably have to go with Super Mario Bros. 3 or Kirby's Adventure.

Super Mario Bros. 3, best game ever made, period.

Donkey Kong Country 2 is my favorite game of all time. Besides that I love the rest of the DKC games, and pretty much all mario platformers.