Dr. Doom/Dormammu/Thanos from the Marvel vs. Capcom series would take all of those universes by themselves. All of them would get along pretty well in other universe's societies as well, becoming overlords or kings and ruling with a merciless fist, and their knowledge of the universes would mean that they would know what to do and how society works, not that it'll stay that way for long once they're done. Dormammu especially would probably be pleased forcing all of the Daedric Princes to bow to him, and Doom would bring inter-Galactic peace and advance Mass Effect technology in ways they probably never though possible.
If that doesn't count because they're originally comic characters then Lucifer from Shin Megami Tensei. He's all powerful, so direct fights wouldn't be a problem, and he's not ruthless. He spends alot of his time traveling through time and just living like a normal person, he's shown, in the 1920's/1930's, that he can fit into a society and remain obscure. He'd probably be a shopkeeper in ES, a nameless scientist in Deus Ex/Mass Effect, or a wanderer in Fallout.
Of course, these guys are completely ridiculous in terms of power output and knowledge so it's really easy to put them in this situation, considering they've all been in it before. Maybe later I'll analyze somebody more... human later, lol, like Leon from Resident Evil.
Out of the specific ones in the poll the Mass Effect universe would prove way too much for Deus Ex/Fallout/and especially Elder Scroll characters. In a fight Mass Effect has Biotics, incredibly powerful spaceships, everybody owns a lasergun, and the technology is leaps and bounds above anything they have to offer. The society is also probably too difficult for them to handle at first, with all of that information coming at them at once. Though, the DE and Fallout guys should be able to adapt eventually.
