What's yours? I won't get into semantics about including remakes or any of that stuff so it's up to you to decide, if it would influence your answer in any way.
As for me, it's undoubtedly Age of Empires 2. I played a lot of Age of Kings and even more when the Conquerors expansion came out. After, I play it on and off and got the Forgotten Empires mod which added new difficulties and civilizations. This Black Friday, I finally got the HD version with all the DLC so I don't see myself stopping any time soon :p
Unfortunately, Age of Empires 2 doesn't track total in game time but each game averages from 1-2hrs and I have been playing the game, though inconsistently, for about 15 years so it really adds up. My guess is 4000+ hrs