The race series is good. Has an okay feel, solid physics, under steers and oversteers when it should, has a lot of mods that can be fun. Worth trying for a few bucks, it's a little dated but great deal for the price. (Race, injection, race on are all expansion packs/the same game) It has fairly good AI for offline racing.
Definitely get the live for speed free demo, or buy it. Probably some of the best street cars, very natural, and has a solid feel. I had the demo and bought the first level ($20) just for the autocross. It is the best autocross simulator bar none. I only use it for autocross but it is great for that! This has a lessons/driving school that is nice as well.
Assetto corsa is considered by many to be the best or one of the best sims available. I like it but it isn't my favorite. It has very good physics overall, and is great for hot lapping and kind of a track day sim. Like a real car it is easy to drive, but once you try to go as fast as you can and are on the edge it isn't as easy. Much better graphics than the ones I mentioned previously.
My favorite sim is gsce game stock car extreme. It is a very underrated sim IMO. It is built off the game engine that ran race and rfactor but is really impressive. I think it has some if the best physics available. Best AI for offline play. First sim that I have had some great offline races. It has a large selection of vehicles from the start that are great plus many mod cars and tracks. The karts alone were worth the $30 price for me.
The last big top contender is rfactor2. I haven't tried this one, but general consensus is that it is like by many. Same thing with iracing. Many will say iracing doesn't feel quite as nice and the physics aren't quite as good as maybe ac gsce or rf2, but overall it has the best online drop in racing. With the other sims you generally want to find a league or group to drive with to ensure realistic and fair driving.
The last one I would comment on is rbr Richard burns rally. If you like rally at all or could, this is a great use of $8! This is a sim, not forgiving but therefore rewarding. Has a pretty great driving school too. Gtr2 is another old sim you can generally get for around $5 or less and was liked by many. I personally don't love it as I always felt the rear of the cars were way too stable under both throttle lift/trail braking, and somewhat under throttle too. I did get it though just for the "driving school" aspect. The reading material on performance driving is very good though.
Hopefully this rambling is of some help. Whatever you decide enjoy! If I could only have four they would be gsc lfs ac and rbr.
Edit: I forgot the makers or gsc just put out a free title called Game Petrobras de Marcas that is a fwd touring series. It is offline only but well worth the download!
