My vote as a seasoned racer
The top three are definitely: Need for Speed, Real Racing and Fastlane (no order at the moment). Do NOT waste your money on Asphalt4, I regret it even at a dollar - buy a song!
Need for Speed offers an excellent story-line that makes you want to play, and a lot of interesting racing methods (smash up cop-cars, normal race, damage things, elimination, etc.) and a fair bit of cars. Storyline isn't too long. Racing is great, though, but a bit controlled.
Real Racing (a bit too expensive, imo) is great, a lot less controlled, great racing, graphics on par with NFS (great), very fun but career mode a little boring as there's no interesting story like NFS.
Fastlane is great, fairly inexpensive and very challenging. Graphics are great (maybe a bit lower than previous two) but gameplay is great, cars are nice, controls spot-on.
Out of all, Fastlane for longest play-time, NFS for replay value and storyline, Fastlane for a fun challenge. If NFS was a fair bit longer with "free-roam" (just drive around like on the console games) it would be the definite winner.