Funnily enough I just reinstalled rFactor which I never really got into before, and nothing has really changed my opinion... I patched it to the latest version and installed the Meganes which seems to be the mod of choice, but wasn't blown away.
I've also been playing a fair bit of GTR2 recently, which of course has the best cars (love FIA GT) but is damn hard. It's starting to show its age a bit, but is without a doubt a great game, and so moddable... worth getting just for Power & Glory alone.
But then after a few days of these two sims I went back to Race 07 and started a new championship with a Seat round Curitiba... and to my mind, Race has the immersion factor, and that's what it's about for me. Most improtantly the feel, the sounds, and particulalrly the FFB in Race is sublime, and you genuinely do feel like you're racing... then for a laugh I took an old WTCC 87 M3 round Valencia in the wet (FANTASTIC), a F3000 for a spin (scary fast .. I'm too old for these... try the FBMWs if you are too), had a blast in a Caterham.. and the Minis are just sublime, so much fun..
So I guess it's Race 07 for me ... and if you're quite new to racing sims, it makes the most sense too.. it works out of the box, has plenty of variety without needing mods... has "easier" classes for a beginner to drive like WTCC .. it's the most up to date graphically .. has a modern UI... can you tell, I like it?