Just a quick grade I gave to each sim.
NR2003: A-
Pros: Everything is good, except some of the driving.
Cons: Needs a good computer to get decent graphics. Pain in the butt to install. Hard to find.
ARCA Sim Racing: B+ Pros: Driving and online racing are amazing, though. Amazing detail, great tracks, and some great addons, like where you can set the race to start at any time you want, or you can set 100 AI. Great sounds. A driving sim.
Cons: needs a better damage model. AI are so stupid.
rFactor: B- Pros: LOTS of mods and addons.
Cons: NOT for stock car racing. Horrible sound pack for VHR stockcar.
iRacing: A-, From what I have seen, this game is great, but could use some work. For instance, servers, offline, LOWER PAYMENTS, and some minor bug fixes could be added.
Pros: Great driving. Detail. Realistic track surfacing, and bumps. Multiple cars and tracks. Great graphics
Cons: TOO MUCH MONEY! Payment for everything. No Offline racing. No servers, runs iRacing's servers. Lots of bugs. Too hard for rookies.