This is a tough one.
But I'll go with Fallout simply because when you break it down Elder Scrolls is another take on the classic fantasy setting while Fallout has that crazy retro 50's sifi motif that colors everything from weapons, to armor and all the way down to those broken TV sets.
Yah have to go Fallout here.
While I enjoy fallout, I find myself enjoying the Elder Scrolls games far more. They are much more interesting to me, not to mention less depressing. [spoiler]Fallout is still great.[/spoiler]
As much as I've enjoyed F4 with it's adaptation of a number of mods from both NV and Skyrim ES takes it. It has an MMO in ESO and I had a fun time playing with a friend via a co-op mod on Skyrim.