Puzzle games are something I find myself going back to the most often, because it's easy to just pop and and play for a while. In that vein Picross DS and Meteos are awesome. Picross has some infuriatingly hard puzzles at the end, but I keep coming back to try and finish them all. I've also sunk more time than I'd like to admit into Animal Crossing. And then of course there's the common ones -- Mario Kart, Mario Party, Pokemon Platinum.