What's your favorite 2D platformer?

Recently dusted off (literally) my old 1000. I like all types of games but haven't delved in to a side scroller \ puzzlet for awhile. I've played a bunch of them with LBP being my favorite. Also up there for me are Shovel Knight games, Rayman games, Super Meat Boy, The Swapper, and Limbo.

I am looking for recommendations, but mostly I want to know:

What's YOUR favorite 2D platformer?