World of Warcraft is the absolute winner in my case. I have no idea on actual numbers, given 99% of my time was in private servers, with a very brief stint on official. Only one of those private servers is still around, with my character from Warlords of Draenor times still there.

Outside that, probably Skyrim (many hours playing pirate versions, modding it with more lethal combat, more lore-friendly armors and "actual" civil war, among other things). I wonder how many hours I've spent on Dwarf Fortress, as I've only played the freeware version, starting with 0.34, before trees had height.