I enjoyed the Mario 3DS, and the sequel looks promising. I have also enjoyed the new Kid Icarus title (although it is no where near as challenging as the original was when I was a kid). I have also been curious to check out the Metal Gear title as that looks enjoyable. Also, my son loves the system so much, he rarely puts it down.
I've heard good things about kid icarus, apparently the controls take some getting used to though. Also there is apparently a new Phoenix wright game coming out soon so naturally I've already preordered that somehow.
Also also here are some great DS games just in case you missed them when they first came out, and since the 3ds is backwards compatible:
Final Fantasy Tactics Advance 2
Chrono Trigger
Advance Wars: Dual Strike
The Ace Attorney Series
Golden Sun, Dark Dawn
All dem Pokeyman games