Mario Kart Wii
Can you name a multiplayer series more fun than Mario Kart? Maybe, but I'll argue with you about it. Mario Kart Wii is not only the best-selling racer of all time, it's also the most fun you can have with four people sitting on a couch. More time has been invested in this game than in pretty much every other racing game I've ever owned combined. It bridges the gap between gamers and non-gamers, is intuitive and fun, and has plenty of reasons to come back and keep playing. Some people complain, but I couldn't be happier.