I'd say Trackmania, but that game, though incredibly fun and deep, comes bundled with the most evil and destructive copy protection software in the history of gaming...Starforce. This POS actually caused my CD drive to fail so hard that I had to repartition my hard drive to get it working again. If you're not afraid of the Starforce though you can get a version of Trackmania for free
here
So...gonna have to go with PGR...the actual racing is pretty bad as the AI is just horrible & most of the tracks aren't really designed with lots of passing in mind, but the Cone challenges are insanely fun and addicting
Honorable mentions to TOCA Race Driver 3 (I hate that this game isn't more popular in the US as it's easily superior to both GT and Forza), Rallisport Challenge 2, and Burnout Revenge.
Midnight Club 2 is fun also...rubber-band AI to the max but being able to pick your own route saves it
Heh, so hard to choose, so many good racing games out there