The Elder Scrolls III: Morrowind
I can even remember the amount of time on my save games for each of my 3 playthroughs a decade ago. First one only got to 90-92 hours on my Rage Fury Pro setup before I had to give up, the card was constantly overheating. My next playthrough was on my 4200ti and the water surfaces stunned me with that upgrade. That and my next playthrough were 140 and 300 hours respectively.
I only picked it up again a few months ago unmodded and it was still spectacular, but I ended up reformatting my PC and finally upgrading from XP. I plan to start it again sometime within the next couple of years, but there's still a lot of games I want to play. I still remember almost everything about it, but never did go to Solstheim. That is something to look forward to whenever I decide to install it again.
The only other games that I've played as much as that are Rome: Total War, X3: Terran Conflict, and possibly Medal of Honor: Allied Assault, Spearhead. Rome and X3 still have plenty of enjoyment potential though.