It’s gonna be hard to pick, because despite seeming like a pretty shallow genre, there are some pretty wild differences in gameplay. Arcade, Simcade, and Simulator all rely on different populations. People who enjoy Horizon are probably not lining up to play iRacing, and vice-versa.

In my opinion, I’d say one of the early Gran Turismo games, specifically 2 through 4. It was many people’s introduction to racing games and, at the time, was pretty cutting edge. That’s now changed with the Sim games, but there are plenty of Sim offerings now and it isn’t necessarily anything “groundbreaking” being released anymore.

I’m hesitant to agree with Horizon because I personally don’t enjoy it at all, but I understand the appeal of the first one, especially in a time where that concept was fairly new.

NFS:U2 Is also up there, due to the era. That was the epitome of late 90s/early 2000s car culture and had a large impact on the scene at the time, and just was a benchmark for the arcade side of stuff for a long time.