Mario is pretty hard to argue with now that it's been brought up. I can also see Pokemon being a contender between the RPGs and the many spinoffs.

Despite being my favorite series I don't know if I would pick TES for this. I don't think there's enough gameplay variety within the series for this particular scenario. They do have an MMO now, but they lack any strategy games, whereas Warcraft brings both the MMO and two (maybe even three) of the greatest strategy games of all time.