I think it will be WoW for me by far. I played a lot on private servers since my 3rd year at Uni, I think it was. Unfortunately all those times I don’t have anymore, however I can remember that 2 of the servers I had 70 days on my accounts (which comes to 1880 hours) and then I played WoW on official servers for the first time last year.
I think I had about 40 days across all my toons on the account, so another 1040 hours. So in total probably more than 3000 hours all time (at least).
Mine has to be Diablo 3. Well over a thousand hours sunk into it, especially with the Loot 2.0 rework after the release on PlayStation.
Warframe is getting there with over 400 hours on the clock in Steam.
The original Guild Wars would be a contender, also with hundreds of hours (if not over 1,000) logged, but since the shutdown of Xfire I’m not sure I have those stats anywhere.
Pre-tracking would be Half-Life mods. Counter-Strike and Day of Defeat kept us busy for hours at LANs in my school days. Call of Duty 4: Modern Warfare supplanted it, but by then the end of my LANning days was nigh.
Baldur’s gate 2 easily, don’t have exact hours but was multiple complete playthroughts.
It must be Caesar III. I played it every day when I was young. It was basically the only thing I had to play. Then we got internet and I found forums for Caesar III and I started experimenting with different blocks and people hosted competitions ie most money by x date. It remains one of my favourite games of all time.