Oblivion, AND SCREW EVERYONE WHO SAYS OTHERWISE. RAWRAWRAW.
Seriously though I've only played Oblivion and Skyrim so I obviously can't judge what the best game of the Elder Scrolls series is, but Oblivion is by far my favourite out of the two. While Skyrim is a great, fun game in its own way, it's just not RPG-y enough for my tastes, I find the landscape of Skyrim dull and the game takes itself way too seriously. Nostalgia most likely clouds my view a bit in this too, but the majority of my opinion on the matter stems from the departure in atmosphere from Oblivion to Skyrim.
Seriously though I've only played Oblivion and Skyrim so I obviously can't judge what the best game of the Elder Scrolls series is, but Oblivion is by far my favourite out of the two. While Skyrim is a great, fun game in its own way, it's just not RPG-y enough for my tastes, I find the landscape of Skyrim dull and the game takes itself way too seriously. Nostalgia most likely clouds my view a bit in this too, but the majority of my opinion on the matter stems from the departure in atmosphere from Oblivion to Skyrim.