rFactor, has to be the most realistic driving simulator available FOR PC. 15 degrees of physical freedom, has it's own physics engine, it's only downside is the fact that (like all other sims) it is strictly race tracks and cars, as well as Decent graphics.
That's at the vanilla state though. With the GTASA-born HDR mod, it looks incredible.
And then, there is Gran Turismo 5.
Gran Turismo sucks, there's nothing realistic about it except for the graphics, and they just use that to cover up the fact that there's NO loving PHYSICS IN IT.
Are you loving nuts!? Gran turismo 4 was compared to a real formula One race with a TWO loving PERCENT DIFFERENCE from real life and in the game! You could pretty much learn how to drive with GT4 and a Logitech G25 Racing wheel!