Fallout for me, for a few reasons. I had played Morrowind perhaps at too tender an age, and then Oblivion again I played while I was on an action game binge. One thing I will always recall about Oblivion is working my first job out of university and this ~45 y.o. coworker says to me 'You like games? Try Oblivion'. I'm thinking -- if this middle aged dude is plugging a game it must be serious.
But Fallout (and when I say Fallout I mean 3/NV, I'm part of the unwashed masses who've never played the originals) has a better mesh with the Bethesda game type in my opinion. Being primarily a shooter takesaway the combat complaints. Being futuristic allows great stuff like audio recordings and radio on your PIP boy. The soundtrack to FO3 is (perhaps I'm being a bit presumptuous here) iconic. Now I know some of the songs my grandparents used to hum because of a video game. I'd never have looked up The Ink Spots on my own. Whereas Ragnar the Red is merely a comedic little ditty I can't even remember the lyrics to, despite it being all of 3 verses long. I'm sure there are some lore lovers who really delve into each Elder Scrolls text they find, but the only reason I ever pick up books is to fill empty shelves in my multitude of houses. I think I've fully read 1, maybe 2 books in Skyrim. But in Fallout I get the recording playing and go about my business while some voice actor dictates the proceedings for me.
Fallout also has the luxury of a great spread of weapons. Energy, Explosive, Melee, Melee(fisticuffs), guns, big guns. The Elder Scrolls on the other hand get magic, which is nice, but aside from that you're stuck with melee or arrows.
For whatever reason, even as someone who typically reads Sword of Shannarah over Ender's Game, I prefer the futuristic wasteland to the medieval setting. I think it has to do with it being post apocalypse 1950's, so it leads me to believe I'm learning a little bit, and it's reminiscent of silver screen movies.
One thing I love Elder Scrolls games for is using the engine to do more than render brown/grey wastes and run down buildings. I love the wasteland, but those mod screens that came out recently show just how nicely Skyrim can portray nature.