rFactor 2 is probably the most advanced and realistic racing simulator. It features day-night cycles, weather, tyre deformation, chassis flex and many more advanced features.
While it may not have the best graphics, the driving experience is, in my humble opinion, the best out there.
You can see this video on youtube.
rF2 has a lot of free content covering cars from all eras as well as many tracks. There are also paid DLCs, most notably GT3, Endurance cars and the Sebring raceway. Free and paid content from 3rd party developers also exists, though some may not be up to the standards of official content.