Halo 2 I probably played more online than I did everything else on Xbox and 360 combined. Although, that's not saying as much as it would seem. Was terrible at it, though.
Monster Hunter Tri Ultimate on the Wii U I played over 100 hours. It may have been as much as 300, but I honestly don't remember. It almost solely made my Wii U purchase worth it. And I might wind up getting a Switch for XX, as well.
I'm into FFXIV at the moment, been playing since they redid it and now the 2nd expansion is about to come out in a month. I don't stay subbed to it, but I'll normally resub everything time a major patch comes out and play it pretty heavily for those 30 days.