Ive raced them all. From the Papy Nascar series to rFactor and even Ratbag. Don't bother with Ratbad LOL..
I have just started with LFS and it feels the most real to me. That being said I love rFactors mods and all you can do with it.
But I got the demo last week just to try it and bought S2 after about two hours of playing with the demo.
LFS S2 is the best sim period as far as physics.