Age of Empires 2. My reasoning would be similar to those who will pick Civ.
Dota 2....Dota 2 never changes.....
I'd go with that because I enjoy it as much now as when I started. My favourites are all single player games but aren't hugely replayable. Maybe I'd go with Dark Souls otherwise.
What if... WHAT IF people stopped playing Dota 2 in favour of, say, Dota 3?