For sim there's two standouts imo:
Assetto Corsa -- good physics (arguably the best, but I don't think there's a huge margin), pretty graphics and a good, and increasing selection of cars and tracks. But limited AI, no multiplayer and with what I've heard of their plans their multiplayer will still be second fiddle to iRacing, imo.
iRacing -- good physics with regular updates (for a 'complete' game, rather than the 'beta' of AC), possibly not quite the best physics but as said I think it's pretty close. Graphics are just ok, I don't mind so much when driving but certainly in pictures/videos it's noticeable. But the best aspect is the online/multiplayer setup, yes it's expensive in general but for non-league racing it's by far and away the best option, nothing else comes close.
pCARS is, imo, poor, graphically it's not significantly better than AC and physics/driving wise it's a step behind both AC and iRacing (and rFactor 2)
rFactor2, admittedly not tried in a while but seems to have the same 'issues' that plagued rFactor, mismatched versions/constant updating of mods, I just can't be bothered with that, I was something to 'just work'.
AC is by far the cheaper so worth picking up, but if you want good close racing at any time with 'randoms' then iRacing is the only option, just make sure you get out of rookie asap
