According to Steam:

1. Rocket League: 2038 hours
2. GTA V: 260 hours
3. Project Cars 2: 234 hours

I haven’t seriously played any of them over the last year though. RL has been run into the ground by greed. GTA V was always greedy, but was still some fun to fck around in. However it isn’t really relevant anymore. PC2 was replaced for me by Assetto Corsa (5th place, 170 hours) which has great mods and better physics.

Also, this is only Steam. Combining Steam and Uplay numbers, Trackmania 2020 probably takes 2nd place. And LoL may also be top 3 even though I haven’t played in years.