As far as 2D Platformers go, Sonic & Knuckles (even moreso Sonic 3 + Knuckles) is my favorite of all time! Truly an incredible game.
However, the greatness of 2D Mario cannot be overstated. Super Mario World is my favorite of the bunch, and his latest entry (New Super Mario Bros. U) can stand among the greats in the entire series. Fantastic stuff.
And of course the Donkey Kong Country series is out of this world as well. Donkey Kong Country 2: Diddy's Kong Quest and Donkey Kong Country Returns are my two favorites in that series. Always tough, but always fair. Much respect, and can't wait for Tropical Freeze.
And for Rayman, my first game was Rayman Advance on the GBA. Really fun game, and I've been a fan ever sense. Rayman Origins was a lot of fun (and a sight to behold), and I can't wait for Rayman Legends. I play the Challenge App every morning (and currently at Awesomeness Level 5! )
For me, these four are the kings of platforming, and I love 'em all. As a platformer fan, I think it's amazing that we'll see all four this holiday season on one console. I can't wait!