Rfactor with the Dirtfactor ad on. I know it's not the best of the best, but if your into dirt racing like I am, it just can't be beat. I raced dirt cars for years and when I saw this video posted a few years back, I just had to have it.
http://www.youtube.com/watch?v=WPFvSawd_rw
I'm now an admin at http://www.teamdtro.com/
It just don't get better than this. http://www.youtube.com/watch?feature=player_embedded&v=96u4uFiWGXg#!
If anyone ever need help getting started, feel free to look me up.