Voted Super Mario 2 Japan as it was the most challenging and one of the hardest games I've ever beaten (straight through, no warps).
Super Mario World and NSMBU are up there though. NSMBU was the most fun I've had with a 2D platformer in over a decade. --- "Everything is bad about the console. It can't play Rugged Stubble Hero 4: Revenge of the guns" -MotiJR on the Wii U