I'd have to say Mario 3, though it's a close one with that and World. Mario 3 still has things that make me scream, but I can't think of a way to make Mario World any harder. (Last thing I did in SMW was getting 92 goals, with the 4 missing ones being the switch palaces.)
-dunno001
-On a quest for the truly perfect game; I don't think it exists...