It's actually quite hard to decide. Skyrim is quite good, and yet...
Actually, it may be Skyrim. Not that I'm blinded by the new shininess of it, but because I find the spell casting in the game much more amusing than VATS or Oblivion's combat. Plus, the dragon fights are something I cannot really think of any other experience in a game to relate it to. When a torrent of flame barely misses my backside while I'm in the middle of fighting Black Mages, and now have a giant mutant lizard killing my enemies while also trying to kill me, it's hard to describe the sense of adventure there.
Bringing up the rear would probably be New Vegas, and yes I realize it'd Obsidian, but it's the same engine as F3. I liked it simply because I was actually able to effectively aim down the sights and snipe in that game, something that was pretty much useless in Fallout 3 because there weren't iron sights. Plus I enjoyed NV's less depressing air. F3 was just all bleak all the time, and though I understand the end of the world would probably be quite a bleak place, after 200+ years...really? How is civilization still so rag-tag?