Meh, I've beaten all of the following, but... Underground 2 was good and took a long time since you had to drive to every race (best bang for your buck), Most Wanted was great (maybe the overall best), Carbon sacrificed a lot for drifting. Pro Street sucked. Since then, I've lost interest and stopped buying the games, just rented them for the weekend which was pretty much all the time it took to either beat or get sick of the newer ones. Undercover, Hot Pursuit and The Run were only okay.
I miss the variety of customizations and tuning and whoever thought adding the race line was a good idea should be shot. (can't remember if they still do this, it came out in Pro Street though) There's nothing more "on the rails" than having to follow a "rail" in order to win. One minor deviation and you can kiss your chances at 1st place goodbye.
i5-3570K, ASRock Z77 Pro4-m, Asus GTX660 TOP, 120 GB Vertex 3 Max IOPS, 2 TB Samsung EcoGreen F4, 8GB G-Skill @1.25V, Silverstone PS07B