I must have put, at least, 400 hours combined into Pokémon Red and Yellow back in the day. I would even say I got 500 easily. I put around 30 or so hours into them every few years, so the count keeps rising.
Diablo II: Lord of Destruction has to be around 400 as well. I played mostly solo, but did team up with a few friends from time to time.
Age of Empires II: The Conquerors has to be around 300 hours or so too. Mind you, I rarely, if ever, played multiplayer.
Diablo III nears the 200-250 hours easily as well.
Most recently, per Steam:
- OG Skyrim: 185 hours.
- Fallout: New Vegas: 140 hours.
- Clicker Heroes: 130 (around 129 idling, I guess)
- Dark Souls: 122 hours.
- Borderlands 2: 119 hours.
- Borderlands: 99 hours.
- Dark Souls 3: 98 hours (I still haven't touched the DLC).
- HITMAN: 93 hours.
- Mafia II: 86 hours.
- MGSV: 82 hours.
All of those in single player. I really don't like playing with others