Minecraft. Despite the copious amounts of AAA software this year and the launch of a new Nintendo handheld (my favs), I can't stop playing Minecraft. I come back to it over and over, and have been for well over a year now. It's amazing, and I really feel like it'll be a timeless game that I still enjoy in 10 years.
Honorable mentions: Pokemon, Animal Crossing, Streets of Rage, Toejam and Earl.