I'd be fine with a 3D Mario, honestly.

Animal Crossing that has features that can be accessed on pc and mobile (screenshot sharing, in game Messanger/mail, ability to order pieces of furniture on the go with your phone for your house, get notifications when people visit/want to visit your town and when new characters/ people are moving in or win a fishing tournament, etc.) I can't even describe the possibilities of animal crossing linking multiple devices together (nx, Wii u, phone, and pc). I'm getting too excited just thinking about it!

Nintendoland 2. 100million guaranteed!

Metroid of course. ;)

Throwing realistic expectations out the window?

Metroid, Super Mario Universe (the biggest Galaxy game with a level editor where you can fly to new, player made Galaxies), Zelda, or Xenoblade all would be a great way to launch a console with a gamer focus and a big title.

NX is a handheld/mobile hybrid.

Angry Birds: Mario Edition launch title.

Oh you're assuming handheld, my bad. In that case Pokemon or Fire Emblem

Putting aside my own preferences (can it launch with, like, three Zelda games? And also F-Zero and Fire Emblem and Metroid?) and looking at this logically, I think 3D Mario is a very strong candidate for a launch title for NX no matter what kind of console it is. The timing is just right. EAD Tokyo can pump out a new major game every 2-3 years. Galaxy 1 in 2007, Galaxy 2 in 2010, 3D Land in 2011, 3D World in 2013. That would put their next game in 2015-2016. It's not a stretch at all to say that Captain Toad and a couple of other minor projects may push that back to 2016-2017.

I'm also pretty confident that EAD Tokyo is working on one of Nintendo's first mobile games. This is a smaller project, not on the same scale as a 3D Mario game, so you wind up with something like 3D World in 2013, Captain Toad in 2014, mobile game in 2015-2016, NX Mario after that.

3D Mario for me. No other Nintendo franchise resonates more than a 3D Mario for me. The Zelda franchise has gotten a bit stale for me considering there have been no major innovations since Ocarina of Time and every subsequent iteration is following the same basic paradigm. Bit disappointed that Nintendo went in the direction of Super Mario 3D World with the Wii U instead of a full blown Mario 64/Galaxy-esque style game. I even loved the direction Mario Sunshine went in. However considering we have not seen an F Zero/Metroid game this generation, I'm not so sure Nintendo knows what is best for them. The Wii U is DYING for a first-person shooter and yet, Metroid is absent. Think of all the cool modifications and customizible options for Samus they could implement into the gamepad, that would actually be worth while use of the hardware, but no, let's simply replicate what is being played on the TV onto the gamepad instead... Sigh..

NX is Nintendo's next home console. Not handheld.