5. Arena.
4. Oblivion.
3. Morrowind.
2. Daggerfall.
1. Skyrim.
Note: This is counting all DLCs, Expansions, and Patches for each TES game.
~Edit~
Oh... As for why?
5. Arena may have my favorite main villain but doesn't have barely any freedom at all with character creation and choices as the other games does. Also, the lack of guild related quests and such doesn't help much.
4. Oblivion is definably the funniest TES game, has an arena, and has some honorable characters however everything is what I call a 'temporally pleasure'. For example, all the good characters ends up dying or leaving in one way or another. (Lucien Lachance, Martin, Sheogorath, etc). And the arena is only a small questline. After that, it is a failure once-a-week type of thing killing animals... I don't want to kill animals in an arena, I want to kill other combatants! Oblivion is also the hardest TES game ONLY if you manage to get to a certain high level and only when fighting specific creatures. Everything else becomes easy and pathetic things to kill.
3. Morrowind definably has the most interesting landscape and best mainquest so far. However, it hosts many negatives for me. Examples? I don't like 'beastly Argonians'. Morrowind is the only TES game I don't play as an Argonian in first cause of that. Not a fan of the luck system and combat system. Cliffracers gave me many headaches. Morrowind is also the easiest TES game and can easily be godly within a week on it with most characters. At least with my experience.
2. Daggerfall is prolly the best at keeping me busy with radiant quests, large dungeons, etc. Also such a long range of items and skills to choose from. Best custom class creation of the TES series, if you ask me. There are many grand things in Daggerfall that I love and very few negatives to say about it... So I rest my case on this one.
1. Skyrim, with the DLCs, is my favorite cause basicly it seems to be a combination of Oblivion, Morrowind, and Daggerfall as well as new and awesome elements. Once again, Radiant Quests are back (Daggerfall) which is always a grand thing for me. Because of Dragonborn, a lot of nice little Morrowind related things had returned like Solstheim, Bonemold, and 'dat alcohol' which looks way much better and cooler than pathetic mead. Plus with the upgraded combat system and some of the nicest armors I've seen and liked, I'm always quite pleased with fighting my enemies in the game which can easily be easy or hard. Depends on how much you abuse the skills, which is all UP TO YOU. I also prefer Blacksmithing over Armorer (They never got Armorer right in the first place. It was mostly an annoyance to anyone using armor or weapons or both). Return of Enchanting, which was something Oblivion was extremely missing. Ability to build my own house is also a million times better than any pre-made houses from main Skyrim game or past games. And many more things that I personally quite liked... Though Skyrim still has flaws. The questlines are quite short, though still decently good and enjoyable. Also, not as annoying as some other questlines from the past were... Etc...
That's all.