rFactor is insane, for sure.
I got used to EA/ISI's old(er) F1 Challenge, and rFactor is much more intense if you're looking for a simulator. They're based on basically the same engine, but rFactor is even more adjustable and mod-able. It's much closer to a real simulation than F1 Challenge was. Also, being the newer game, rFactor tends to have more eye-candy.
For now, though, I play F1 Challenge more, because I'm just more used to it. It's still tons of fun, and it doesn't look half-bad with the mods available today.
GTR2 is also a lot of fun, but if you're used to F1 cars, they're quite an adjustment.
It's also quite good-looking, and the cars look pretty lusty as well.
Test Drive Unlimited is new to me still, but as I haven't progressed too deeply into it, the game is decidedly more "arcade" than "simulator." TDU is quite pretty, though; however, until I unlock more of the sim aspects of it, I can't really say it's as fun for me.