Measured on Steam:
1. Crusader Kings 2: 902.8 hours, probably far more in reality. I had it pirated for a while.
2. Euro Truck Simulator 2: 480.4 hours.
3. Mount & Blade Warband: 396.2 hours, but again, many more hours before I bought it legitimately.
4. American Truck Simulator: 237.6 hours.
Unmeasured:
1. Football Manager 2007. Played that for like a decade after it came out. My guess is the number of hours must be over a thousand.
2. Morrowind. Could be my overall number 1 or 2 actually. Hard to say for sure now. Assuming quadruple digits too.
3. Civ IV. Actually, the time I spent playing Earth 18 Civs could be a category of its own.
4. Europa Engine Paradox games back in the late 2000s/early 2010s: HoI2 Armageddon, EU2 (later For the Glory with AGCEEP), CK1. Nowadays I don't play them much anymore, but collectively they must run into thousands of hours.
5. Pokémon Red on emulator and Yellow and Gold on GBC back when I was like 12. Must have been a massive time sink as well, but hard to estimate now.
Other games that I used to play the shit out of back in the day, and/or repeatedly kept coming back to: Jagged Alliance 2, SimCity 3000, GTA San Andreas (and 3 and Vice City to a lesser extent), Vampire Bloodlines, Deus Ex, Anno 1602, FreeSpace 1 and 2, Age of Empires II, Rise of Nations, Star Wars KOTOR 1 and 2, Sid Meier's Pirates, Tropico 1 and 3, Heroes of Might and Magic II, III and IV, Final Fantasy I-VII. Here it gets a bit murky, and very hard to put a number on it. But these are all games I've replayed multiple times (or kept playing repeatedly, as some of them are more open-ended/sandbox), so each of them must be in the lower range of triple digits of hours.
Just thinking about the numbers, and the love for repetition shown by them in combination with the type of games, I'm glad I never got into MMOs. I seem to have the personality to get addicted to them, but it never happened.