I got into the TES franchise when Oblivion came out, and I loved it! One of my friends convinced me to try Fallout 3 - I was quite hesitant, I thought it just looked like dull space marine shooter at first glance. But when I'd played it for roughly a couple of hours, I was convinced it was the best game ever - and I still am.
So for the atmosphere, the setting - with it's immensely original blend of post-apocalyptic sci-fi and 50s pop culture (way more original than the world of TES, and the humor and references to modern society (not much of either in TES in my opinion) I'd definitely have to go with Fallout.