Got my 3DS, been hooked on it and rotating between the 3 games I own...
Mario Kart 7: Just Mario Kart! Can never go wrong with it. Getting used to the controls, though.
Donkey Kong Country Returns: NERD RAGE! So fun, but so stressful at times as well. As with MK7, still getting used to the controls.
Mario & Luigi Dream Team: I wasn't too sure what to expect with it, only about 15-20 minutes in, and it is SO FUN! I am guessing it'll only get better