This was written as an answer to a beginner’s question on reddit. After writing it, I found it could be an article by itself here.
tl;dr: Get Assetto Corsa on sale, enjoy graphics, physics and mods, get Automobilista to have a lot of (south)american tracks and cars you never heard of before, get rFactor 2 to get night racing and weather. Your choice.
If you want realistic physics you have three choices:
Assetto Corsa: Looks very good, supports VR, has Metric TONS of Mods so there’s a very low risk you’ll get bored soon. Right now it’s on sale on Humble Bundle.
rFactor 2: Looks “gamier” than AC, the physics simulation has the extra that the developers have done a lot of work in simulating tyre physics, supports VR (at least in the beta DX11 version), has not as many Mods as AC but still quite a lot with the bonus you can install the mods directly thru Steam Workshop. Has night and different weather conditions (AC does not)
Automobilista: Initially based on the same engine rFactor 2 was based on (rFactor 2 developers have changed the engine a lot since then) so you’ll have at least as good physics as in RF2, does NOT support VR out of the box, Automobilista has more mods that rFactor2 but less than AC, and it has many tracks and cars you find in (south)America that the other Sims don’t have.
I like them all three and race them regularly. It depends on what you prefer. AC looks best, and has the most mods by far. Automobilista has a lot of cars and tracks none of the other two have. RF2 has the better VR support and night and day races with different weather conditions.
You may want to get AC and it’s DLCs from the Humble Bundle and wait until rf2 and automobilista are on sale.
Any further questions, feel free to ask in the comments or on our Forum!