Everquest. Easily over 10,000 hours. Played it pretty solid for 15 years. Almost every single day from 6 pm or 10pm or somewhere in between until 3am or until I was near about to fall asleep. Some days I would play starting even earlier, like in the morning till next morning. Though that was only the first 7 years or so when I was doing college and only working 10-20 hours a week.
The second game with the most hours would probably be Europa Universalis IV currently at 2683 hours starting from June 19, 2016.