Luigi's Mansion 2 (Luigi's Mansion: Dark Moon) blew me away. It's so effortlessly charming that it's hard not to enjoy every moment of that game. I can't remember a game that funny and lovable.
Resident Evil: Revelations is pretty damn good, and it's Raid Mode is actually better than Mercenaries in my opinion.
Mario Kart 7. The best Mario Kart ever made, and the best handheld racer I've ever played. The online is solid - complete with races that disable the Blue Shell (!!!) - and it's 60FPS 3D is gorgeous.
Super Street Fighter IV. The best fighter of all time. In your hand. In 3D. What more can I add?
Star Fox 64 3D. The re-make is superior to the original in every way - yet it also includes a "Nintendo 64" mode that ensures it plays EXACTLY the way it should. It's fast, clean fun!
Super Mario 3D Land. The first half of the game is simple fun - the second half is the most challenging 3D platformer I've ever played. It's punishingly difficult in comparison to what's come before. It's inventive, yet it feels familiar. If you like Mario, you'll LOVE this.