Hmm tough choices
Super Mario Bros. 3 has to be the end-all be-all for me though, that to me is Mario personified in digital format. The classic plot, strange enemies, lots of fun ways to bring out game genie glitches/quirks, and challenging yet rewarding. Oh, and who can forget the wind-up boot!! HAH, I GOT THE SHOE!!! I GOT THE FREEKIN' SHOE!!
Super Mario World comes in second, I find it a little too long to just pick up and play whenever I want to, I often end up setting goals to pass. I often have to scurry back downstairs in 3 minutes if I start playing SMW at work.
Third would be Super Mario RPG, same deal as world, I just get to immersed to make it a short term run. I'd have to spend hours upon hours entrenched in the storyline as if I were Mario myself, that's just how I play, it's an escape.
Fourth is Super Mario Land 2: The Six Golden Coins - I spent hours playing this on school trips, day camp trips, and during the bus rides home from school. This is one of the rare few Game Boy titles I know in and out and through and through.
And Finally is MARIO PAINT! Sure, it's not a game, but by god, a piece of creative software for the SNES in a time when a decent SVGA equipped 486 computer with Windows 3.1x cost $3,000....thank god for Nintendo. I spent hours and hours drawing, animating, playing the bug swatter game, and composing on that cart with the little SNES mouse. Shame I can't find the mouse for my copy that I have now.