One of those gosh-darned Pokémons. Especially the ones where I bred competitively viable Pokémon. So it's one of Omega Ruby, Sun, and Ultra Sun. I'll check back on their play times when I can.
I've also played a lot of The Sims, The Sims 2, and Stardew Valley (183 hours). I'm not sure how to get the exact play time for the first two, so I'll just leave them mentioned as I did.
If Pokémon Showdown counts as a game, then that would be a real contender as well.