Assetto Corsa Competizione - I can just jump on and hit the multiplayer button and then just have a quick look at the servers or hit the quick match to jump into something.
BUT - I do find my self jumping back out of the Quick Race if it's set up something like
40min Practice
20 Qualifying
15 Racing
I don't get that sort of server setup at all in a quick race setup - it's in the name quick race, not 60min and then 15min racing.
For me the sweet spot in something more like 15min Qualifying and 20min Racing
This may change in the future as I'm building up my SA rating (currently 67) so I can unlock the other matches.
If it's not online racing then AMS2 or RaceRoom are great as the AI in those games are interesting to race against.