Assetto Corsa, of course. I have put over a hundred hours on it, and I rarely play any game that much. The reason is simply the amount of fun it is to drive a car in Assetto Corsa. It's realistic and intuitive on a steering wheel. Driving a Ferrari really fast has its own draw, as does getting better and better lap times and driving different cars in various tracks. There will be more content coming in free updates and in paid DLC, and knowing how good the cars and tracks have been so far, the game will only get better. There are also some pretty good mods for it.
Number two
Wreckfest, which was previously known as Next Car Game. It's very much the opposite of Assetto Corsa: silly, arcade, off-road racing. It's pretty easy to play, and playing with a steering wheel is great. It's in Early Access and doesn't have much content yet. Flatout had a great single player mode so I don't see why wouldn't Wreckfest have one too. I just don't know how long will it take for the devs to make updates or release the game.
Number three
If the question wasn't specifically about racing, number three would have beeen Euro Truck Simulator 2. It just is surprisingly good in a weird way.
But since the question is about racing and I can't make up my mind, I'll say Race 07 / Live for Speed. LFS was one of the first games I played with my G27 steering wheel, and the driving was awesome. But the sounds and graphics were really bad and it didn't have real cars or tracks. So I bought Race 07 along with some DLC and I was pleased. Driving was almost as good as in LFS, the audio&video were better, but best of all, all cars and tracks were licensed.
When I found out about Assetto Corsa, it changed everything. It has the best handling, all real cars and tracks, as well as excellent graphics and sound. I didn't even play racing games that much before I bought the G27, but now AC is one of my all-time favorite games.