Forza Motorsport is easily one of the best racing games ever. It's fun, and customization is insane.
The physics system is easily one of the best for racing games.
I so can't wait for Forza Motorsport 2.
Gran Turismo HD has nothing on it (considering that it's basically GT4 reprogrammed to use the PS3's hardware). Until Polyphony Studios releases Gran Turismo 5 in three to four years, it's all about Forza Motorsport 2 baby!
Maybe Project Gotham Racing 4. The X06 trailer did look pretty nice.