Definitely WoW overall, but I don't know how much time. Minecraft is around 300 I think. Factorio is a fairly recent addition for me, but is leading my steam library now, with Terraria in second. I assume RuneScape would be a contender as well. Maybe ESO too... I used to have a bit of an MMO problem.
Man, years ago I grabbed an add-on for WoW that made a database of everything across all characters. It tracked gold, items, etc, and playtime.... I was not prepared to see i had over a year of playtime.... Which granted, includes AFK and time in queues, but still.