Uh, if I had to choose, it would be a toss-up between Automobilista and Assetto Corsa. I probably would choose Automobilista because I've spent more time with Automobilista more than I have Assetto Corsa. Having a less-than-stellar PC is a factor for me not playing Assetto Corsa much. AC, though, still is one of the most fascinating simulation racers I've ever played. The first Automobilista is based on the rFactor 1 engine, so I was able to play it fairly well and keep playing it.
Honorable mention for me would be "BeamNG.drive." It's for the same reasons I haven't played Assetto Corsa too much, and BeamNG.drive is even more resource intensive than Assetto Corsa.
So my vote for 2010s racers is Automobilista.