Which is your pick for #1, and why?

NSMBU in my opinion. I haven't played NSMB 2 though.

New Super Mario Bros U because it basically took the best and made them better! And New Super Luigi U came as dlc which was also pretty awesome!

U because it's HD and has DLC.

Definitele Super Mario Bros U. It's also the first time yoy can play as "Luigi" in Luigi U.

Well technically I'd say NSMBU even though I didn't really enjoy it. NSMB2 had just come out and even that let me down. For nostalgic reasons I'll vote for the first one (but it controls very weird compared to the newer ones). Also it has better level design and bosses.

Edit: I did love one thing about NSMBU, and that is NSLU! Also, I don't hate NSMBU, but it's way too much like NSMBWii. The unified world map is neat though, and so are the controls. 

Why so many votes for U? I found it quite dull.

I much preferred the Wii version...

NSMB Wii... the magic was back in full for a home console since Mario Bros 3...

NSMBU is great and truly amazing but came a little bit too close to NSMB2. I had just finished to enjoy one that start a new one felt a bit too much... and that Luii DLC just happened to make it near perfect... if only was released a few months later!!!

New Super Mario Bros., because it's the only one that truly was "NEW" when it released. The other 3 should be called "Lazy Super Mario Bros. Wii, 2, and U." This series makes me so angry, because the old 2D Mario games were incredible. These are so half-assed and lazy with the visuals, the music, the level design, and just everything. They're lazy. Lazy and dull. That being said, the handheld ones are a little more "acceptable" and "passable" in my eyes because expectations are lower for handheld iterations. Plus the camera is zoomed in closer. In Wii and U, the camera is so zoomed out, and the levels feel so "empty" because they're designed to accommodate 4 players. Unfortunately, the multiplayer in those two games is horrendous and frustrating. So, you're left with an "empty" zoomed-out single player experience, or a frenetic, spastic, irritating multiplayer mess.

3D Mario for me, please. Thanks.

From what I gather around the interwebz, its generally considered better thanks to its unified world map and greater challenge.