I've played most of the NFS series, Forza 3 & 4, Gran Turismo 3, 4, 5 and iRacing.
iRacing and GT4 / GT5 are the only ones you could possibly call "simulators". The braking sensation is much better in iRacing when compared to GT5, but I like the way GT5 feels when the car is shifting it's weight. I find the cars slide around too much with iRacing, but the New Tire Model last week was a step in the right direction.
For a pure racing sim, it's iRacing. For something that's 95% as close, but much more fun (IMHO) and easier to use it's GT5.
Next would be the Forza 4 & the NFS Shift series, but they are a BIG step back from GT5 and iRacing. I played Forza 4 on hard, all driving aids off, with a controller, and I was easily able to drift a damaged 458 around a turn at 200kph. It was too arcade-y for my liking. Then at the end of the spectrum are the other NFS series, "Most Wanted / Underground / etc) which are
a small step above pure "arcade" games.
Steven Morris wrote:
To "complete" Forza 4, takes a significant amount of time. I am only 14.5% through all the events and I have 35+ hours of driving time (the game keeps track of all these stats). It is starting to feel like a bit of a grind though.
You don't have to "complete" GT5 to have fun, just the license tests so that you can drive any car you want. That only takes a few days. However, I'm 99% "complete", and I've driven 170,086km. (Even at an average speed of 200km that's a lot of time).
Keep in mind there are challenges in GT5 like the 24hrs of Le Mans and 24hrs of Nürburgring, which don't need to be completed, but I had a lot of fun doing them with friends.