Fun topic. I chose Super Mario World.
Mario Fav. list:
1.) Mario World
2.) Mario 64/Mario 64 DS
3.) Mario 3
4.) Mario Land 2: Six Golden Coins
5.) Mario USA/Mario 2.
*"Mario World" felt the most balanced and complete.
*"Mario 64" was a revolutionary title and is still an unbelievably fun game to play. You really feel the freedom of 3D and Mario's athleticism in it. The only tiny nitpick I have is that it could have had a little more iconography (which the DS redux began to move in the right direction to fix with the addition of stuff like the P-Balloon ability).
*"Mario 3" is just plain crazy. So much new ground covered and it explodes with creativity. The level designs are often a little too short, though, and it just doesn't feel as balanced as Super Mario World.
*"Mario Land 2: Six Golden Coins" is often overlooked, but it's a real gem. It feels fresh. Plus, this is when Wario debuted and was still a cool, competent villain. (I hate what they've done with Wario over the years. He could have been a great reoccurring boss like the Koopa Kids. Now, he's an oaf).
*"Mario USA/Mario 2" may not have been a Mario game originally, but it's as Mario to me as Mario 1. Why characters like Fryguy, Mouser, and the rest haven't reappeared as villains since is beyond me. They were awesome. And those potions that created doors...incredible.
Those five are my essential Mario titles. The others, while generally solid, I could take or leave. "New Super Mario Bros" was decidedly average despite its good graphics. That whole "giant Mario is new" bit that they sold the game on turned out to be false, when I realized they had already done it in Mario 64 DS in 3D. My verdict's out on "Galaxy," as I don't have a Wii. The only other thing I want to say is that "Super Mario World 2: Yoshi's Island" is really overrated.