Well, since i don't yet have ANY 3DS game (even though i've had a 3DS for quite a while) i cannot properly judge, however, from what i've seen on gameplay videos, i have a 4-way tie between Mario Kart 7, Super Smash Bros. For 3DS, Kid Icarus Uprising and Fire Emblem Awakening
Why the 4-way tie? Well, each game has it's reasons to be my favorite (or, more accurately, my most wanted)
Mario Kart 7, due to it's inmense replayability, and actual online activity, lots of people play this game, so it could actually be quite fun.
Super Smash Bros. For 3DS, same reason as MK7
Fire Emblem Awakening, due to the amazing characters, storyline and gameplay, as well as the inmense amount of content of the game, what with a bunch of DLC and non-DLC maps to be played and all, this would be the game to play when there is no wi-fi connection available, since it is almost completely single-player
And last but not least, Kid Icarus Uprising, due to a mix between SSB and MK7's online playing, and Fire Emblem Awakening single player mode. That, and the fact that gameplay looks fairly intense and fun.
In the end, i can't decide :/ (so i might as well start getting play coins... I should at least get either SSB or MK7 first, there are lots of contests aroudn these right? I could gain more prizes with these...)