Mario Kart one thousand times over. It's (in my opinion) one of the few ongoing racing series that hasn't made a bad title.
F-Zero is also a fantastic series (which i'm surprised no ones even mentioned yet). High speed racing with winding tracks, jumps and loops a long with a kick-ass soundtrack? Yes please.
Finally, Midtown Madness 3 for the original X-BOX. This game was very fun, featuring a lot of varied cars and paint-jobs. It also had a very nice story mode, but to me, the best part of the game was cruise mode. You got to drive around doing whatever you want, including police chases, car soccer, driving through the subway and jumping the river Seine.