The best racing simulation, period.
The 360 since launch has had great racing games but this is the one to own. Many people will be turned off by the highly realistic physics but give it a chance. This is easily the best racing on the 360 and it will be for a long time. There are a ton of cars all of which can be upgraded, tuned and customized with paint schemes and body kits. If you think you have the talent make a custom paint job on a car and sell it over XBL. There are plenty of races to keep to busy and then if you do manage to win them all theres online races. In the end this is the best racing game on any console and definitely one of the best games on the 360.