World of Warcraft

There's a reason I've not only spent more time playing it than any other game (possibly combined, by this point) but played it continuously for around 5 years (with the occasional 1-2 month, maybe 4 month break)

I'm honestly surprised no one else listed an MMO. A majority of people who play them have spent more time playing them than any other game, and generally continue to play them to this day.