Monster Hunter 3:Ultimate. It was the main reason I decided the 3DS was worth getting, although when I tried it I decided I was a bit frustrated with how the swimming element worked. Monster Hunter 4:Ultimate was way better though, and since it was online too I was really glad I bought the 3DS. There have been a few other decent games I've been happy to play, of course, such as AC:NL, but have to say that Monster Hunter is still the highlight for me.
I bought the 3ds at launch bc i knew Nintendo was planning release the AC for this console, but my first game (and i didn't like it) was Rayman 3D. The second one was Nintendogs+Cats Toy Poodle. And the third one was the ACNL, i bought it at launch.