Man, I've come to realize that if Donkey Kong : TF was an option, that would have been my number 1 choice easy. As it stands, I'd have to say Zelda simply because in the past year, I've played NSMB Wii, Super Mario Galaxy, NSMB 2 and Super Mario 3D Land. Whereas, I haven't played a Zelda title in over a year despite downloading Oracle of Seasons and Ages. Plus, I still have yet to crack open Skyward Sword. So, right now, it would be the fresher experience.
Heck, I might have been more tempted with Mario Party if there was some kind of Online mode but as it stands, I have no local friends with a 3DS and I get along fine with the older versions to play instead if we want to play such a game.