Sim wise iRacing, as mentioned, is top of the pile, I've been a member there since 2008 just before the public opening and own all the content, over $1000 spent but well worth it imo.
There is also:
rFactor 2 which is ok, for me the lack of an online system like iRacing's ruins this for me, bought it and not been touched in a loooong time (in fact price-per-hour iracing probably beats it, even at many times the cost

)
pCARS, pretty car graphics, average at all other graphics, mediocre at best 'feel', not a fan.
Assetto Corsa, could be awesome, the netKar physics are extremely good, and it looks stunning, just need them to actually release the thing, and then 'finish' it, netKar's biggest problem was that it was never really finished, so a bit of history from that developer...
Then there are the older, but still 'ok', and generally very cheap sims:
Race '07, GTR2, LFS, rFactor 1 etc etc
But that's if you really want a sim, lets face it Forza and Gran Turismo are not sims, a sim will suck up months of your life just practicing to get even semi-competitive, and then you'll have some of the best/closest/nerve-wracking racing to be had in a game, and/or some of the most frustrating races
