GTR2. Without any doubt.
Mainly due to the base Sim engine.
Mainly due to the vast number of mods and tracks.
Yes, there's a tad more in rFactor, but I prefer SimBin's way of defining 'sim'. Moreover, it's quite easy to finetune mods, mixing classes, setting up personal championships and so on. Which includes the family WTCC, Race07, STCC, STCC2 and in some manner RaceRoom, too.
Speaking online, I've invested lots of hours with SimRaceWay and iRacing a bit as well. I really like the sim engine in iRacing.
Assetto Corsa I like some mods, too, but I think it needs further polishment before an official release, the sim part feels too much 'Grid' like with a touch of NFS Shift. Not my style.
I really hope the ppl at Slightly Mad Studios copy some of their SimBin fellows work talking the sim part of upcoming Project C.A.R.S.
Speaking rally, I still play Colin McRae Rally 2, yes It's old, but the basic rally sim part engine was done that time, though I enjoy playing the latest WRC releases and Dirt-series, too.
But I like the retro part just as I really enjoy unearthing my Speed King for C64 once in a while - my first racing game I would call 'sim'
/Edited due to some nonsence auto-correction from my new smartphone in the original post.
Edited by stenovitz, 06 December 2014 - 10:59.