For offline games I looked at sheer hours played over years and years, I'd have to say, hmmm

Probably Baldur's Gate (II more than I)
The Dragon Age franchise
Anno 1402

Though it's possible Skyrim has caught up to these as well.

For online games, WoW is probably still very much in the lead, even though I stopped playing years ago. I played that game almost every day for years. I play ESO nowadays, but much more sporadically.