Hmm, tough to say, but here's my piece:
1) Animal Crossing: New Leaf - I don't think I was addicted to any other game like this in quite a while, pretty simple, but fun.
2) Super Mario 3D Land - This was my introduction to the 3DS and it was a ton of fun, can't wait for this mechanic to be used on the Wii U.
3) Pokemon X/Y - Lots of addictive gameplay and it's the RPG where you can basically choose how involved you get.
4) The Legend of Zelda: Ocarina of Time 3D - c'mon.
5) Luigi's Mansion: Dark Moon - Really enjoyed this one quite a bit. Very inventive and fun with some good challenges inbetween.
6) Mario & Luigi: Dream Team - Still haven't finished this one, but something about this series is a lot of fun. I even like the grinding. The one part that drives me nuts is the constant amount of hand holding in the beginning...but it's gold after that.
7) Starfox 64 3D - This is a great game to just sit down and play without too much thinking...and you can easily beat it in one session.
8) New Super Mario Bros. 2 - Lots of the same, but that's not a bad thing. It's great Mario platforming.
9) Mario Kart 7 - Another good game to pick-up and just play. Online is great, probably some of the best the 3DS has to offer after Pokemon.
10) Theatrhythm: Final Fantasy - I enjoyed this one, it is a rhythm/music game and might be less interesting if you've never played any FF games.