For a single game it's Factorio. Steam has me at around 1600 hours.
For a series it's gotta be Civilization. I've been playing since I was in fifth grade. I'd go over to my friend's house when he wasn't home to play Civ I. I'd spend entire weekends in college playing Civ II and III. I had to have my GF hide my Civ V disk so I wouldn't miss deadlines. And now I'm on a full playthrough of all the new civs in the latest DLC.