For exploration I would put Morrowind as number 1.
Morrowind has the largest world between morrowind/oblivion/skyrim of course Daggerfall was even bigger, but I don't choose it cause it had randomly generated content and not a true world design.
So for vast world to explore and roam around it wins hands down. Those that never played the previous just check it on wikipedia also they compare world sizes.
For Quests and rich stories and characters / drama / emotion, I choose Oblivion. Oblivion is so far to me had a more epic story and the side quests themselves felt epic at times. And the random conversations in town ranged from funny to creepy.
for graphics, animation, combat I give it to Skyrim (Minus the user interface!) If Skyrim would have been designed for the PC first and ported to consoles it would have been a much more epic game. But with the PC being a console port, the keymaps, the user interface and such were just cumbersome and clunky and didn't make sense.
hopefully in the future we will get an Elder Scrolls game that combines the HUGE world size of Morrowind, with the characters stories, drama, random interactions, quests, emotions and deep rich story of Oblivion with the better graphics and smooth character animations and better combat system like Skyrim.
That would be, from my perspective, the perfect Elder Scrolls game.