I've played all candidates except Shovel Knight, and from them I would pick NSMBU. Still impressed with the multiplayer added on the Wii, plus that connected overworld is a joy to discover the shortcuts. And was surprised by the challenges with consecutive jumping/bumps sections in there - even the gamepad ones!
Rayman's gamepad is smartly used, but it's non-existant on Donkey Kong. It's nice to see Stealth Inc 2 pop-up a few times in the thread, been thinking on getting it; although I must wait for the mandatory eShop sale!