Luigi's Mansion was great, the only problem with it was its longevity. You can barely beat it in just a few hours. Dark Moon promises something big, has some great puzzles on it, and the best and wackiest animations I've ever seen on a Nintendo game. Even multiplayer looks promising.
Mario Golf World Tour looks great, I'm not a big fan of Mario Golf, but this one has incredible graphics for a 3ds game, and even some good escenarios, like the garden with a gigantic Goomba walking around.
The newest Mario & Luigi looks... weird. You basically just touch Luigi while he's on his sleep (yes... it's difficult to explain in the proper context). But who knows, everybody was criticizing Bowser's Inside Story when it was announced just because it looked too different, but ended up being an excellent addition to the series.
Ah! and if you haven't played Donkey Kong Country Returns, this is your great opportunity to purchase it for 3ds, guys! I just watched the trailer in 3d and it looks outstanding. I have the wii version, so I'm probably not getting it, but it's awesome that they're porting such a great game from Retro Studios.