Witcher 3 at 250 hours is my max. That was after a playthrough, then a full (game + both DLCs) playthrough and one last playthrough of just Blood and Wine.
MGS V is the most impressive for me: a single playthrough took 120 hours.
EDIT: as I expected, those are on the extremely low end of the scale for here. I might get around 500 hours for Heroes of Might and Magic 2 but I have had that game since 1996!
WoW would win. Haven't played in 6 years, but I used to play a minimum of 5-8 hours a day every day for a good 3 years not to mention a couple of other years were it was more casual. So likely at least 3000+ hours?
Just checked and Diablo 3 has 1043 hours on it.
Battlefield 3 and 4 had a combined 887 hours. Over 500 on just BF3.
I'd assume it was either Phantasy Star Online on the Gamecube or Minecraft across many different platforms. Binding of Isaac is probably up there. Resident Evil 4, as well. Maybe Super Mario World since I've been playing that one since I can remember.
-Halo 3, Reach and 4 (Bet I played a grand total of 2,000k together here, Reach taking the lion's share)
-Oblivion and Skyrim
-Breath of the Wild
-Phantasy Star Universe (360 'free' demo version)