While all are very good games, I can’t help but suggest Real Racing 2 over any of the other ones. It contains real, licensed cars, GameCenter achievements, and online multiplayer. The graphics are by and far superb compared to its predecessor, and it’s a complete racing experience that just cannot be duplicated. Real Racing 1 is a solid game as well, and for only $1.99—even though it’s over a year old now—is a great steal.
The sale will run until February 14th, so I suggest you get buying before it ends.