Depends on what you want.

I mean, if you want simulator, probably the best is Gran Turismo series.

If you want arcade, i have to say Tokyo Xtreme Racer Zero, Tokyo Xtreme Racer 3 (like Zero but better graphics, in ps2) and NFS Underground.

I almost forgot to put the Motorstorm series, this is arcade too, but just too awesome.

Grettings.