Basically, your favorite and why. And I'm not counting TES-O since it isn't out yet.
As for me. I may've started with Morrowind but after playing almost all of them (Didn't play any Travels, Redguard, or Battlespire)... I say Skyrim is my favorite.
Why? Well. After tallying up all the pros and cons of each TES game, Skyrim has the most pros and least cons for me... I know the other TES games had done better than Skyrim in some aspects but overall, Skyrim wins for me.
Now. What about you?