Where I have a good idea of the figures:
Skyrim: Steam says 1314 hours.
Morrowind: a simple calculation I did after playing pretty much nothing else for two and a half years suggested around 2000-2500 hours.
Diablo2 might just beat that, but it's close.
But top of them all is Nethack, especially v 3.0 and 3.1 - mainly because, for the whole time between 1992 and 1998 it was about the only game I had. Work was slow, so I played it for hours there (my marathon best was seven hours continuous). My PC was so slow nothing else would play on it. The replacement PC wasn't a lot better. It wasn't until I bought a machine I could play Doom on that I was finally weaned of it. But I still go back now and again, because it's still about the most complex game out there.