Probably Overwatch and Forza Horizon 3. Multiple 10+ hours sessions with both games. I'm not exactly sure but maybe the same can be said for Fire Emblem Fates.
Either way, one of the above games takes it. I can't say enough how great 2016 has been thus far.
Playing: F1 2016 (PC), Uncharted: The Lost Legacy (PS4)
Watching: Hajime no Ippo, Attack on Titan (4th time)
Upcoming purchases in October: