WoW easily, though not stacking those hours up anymore. Played a LOT from roughly the release until WoD where I slowed down before quitting outright during Legion due to my disliking the value to cost ratio. It was also, what ultimately made me leave the MMO market entirely despite there being plenty of alternatives with fairer monetary systems in place.
However, coming in second without much of an equal in of itself, is Minecraft. I've got ungodly amounts of hours poured into that game and still, to this day, pumping more into it through my Switch system. It's just a timeless, fun as hell game and is the strongest evidence as to why more money does not equate to a better product and helped to further solidify my feelings on value versus cost. The latter, ironically seeming to lower the former even as it climbs. They sadly do not seem to correlate much if any, just the opposite.
After those two? Hard to say. Maybe Counter-Strike: Source.