While Bioware completely and totally wins the award of the two for best narrative I thing Bethesda truly accomplishes the "role-playing game" aspect of RPGs. While games like Mass Effect give you a compelling story and are also fun to play, I think the direction that was taken with games like Oblivion and Fallout 3 gave you a role and said play it. That was it, they truly let you play a character in a world and perhaps that's not what "RPGs" are. Perhaps "RPGs" are about playing a particular role, but that's not how the phrase would be defined if you looked at it away from the acronym. Bethesda's games put you in a world and let you experience it; be it a dangerous or safe experience. Where Bioware's games give you a role to experience in a world you can do little to affect in the scope of things.
I will say that I personally think Mass Effect and Mass Effect 2 were superior games to Fallout 3 and Oblivion but in terms of defining the genre, I think Bethesda made their games better.
I will say that I personally think Mass Effect and Mass Effect 2 were superior games to Fallout 3 and Oblivion but in terms of defining the genre, I think Bethesda made their games better.