If the OP is assuming we were or became part of the world and we not constrained by the limits of the game then I would say the Mass Effect universe. It's got the right mix of old and new, good and evil - same shit, different day but with knobs on. There is also a level of civility and security in most of the populated areas and hubs where someone like me, who has all the upper body strength of a chaffinch and the combat skill of a dead rock, can walk around, explore, learn and interact in relative safety.
I might even meet Liara! One can dream...
As much as I think the worlds of Fallout 3, New Vegas and Skyrim are rich, wonderful and interesting in their own ways, someone like me would be chewed up and spat out inside half an hour. I'd probably also crap my pants the second I ran into a feral ghoul or super mutant - just sayin'...
Having read the OP again, I am starting to think that he meant worlds I like to explore and poke around whilst playing the game but leaving the plot and side missions to one side while I do so. If that is the case then the ME universe, while incredibly rich, diverse and detailed, is somewhat limited when it comes to exploration.
As if to mirror my original answer, I do love to roam the Capital Wasteland, New Vegas/Mojave and Skyrim, as there's tonnes to see, do and discover. In addition, I often find myself going back to Zion Canyon (Fallout New Vegas DLC, Honest Hearts) and having a good wander around as if my character were on holiday, so to speak.