It's a difficult one to answer.
Like for example lots of people like the Grid/Dirt games. They're probably the most popular racing games on PC currently. But I hate them cos they have horrible arcadey physics models and the cars steer from the centres and it just feels wrong.
The beauty of Flatout 2 is that it struck a balance between a *convincing* physics and driving model and *fun*. There are plenty of arcade racers and there are plenty of racing sims but for me they both miss the point. FO2 was like convincingly modelled physics but you raced on the moon.
The best racing sim currently is probably
Assetto Corsa. Also of note is
Project Cars.
For arcade real-world racing there's really only Dirt and Grid, everything else is even worse.
The most fun I've had in a racing game lately though has been with
Sonic & All Stars Racing Transformed. Which is kinda like Mario Kart but way better. Had enormous fun playing that with friends.
Then of course there's the Trackmanias. Of which I think
Canyon is the most fun.