Like other have said, morrowind for the setting, the lore, the story. Can't ever forget that game, the places you go to, the perils you face. It took me 10 years after i bought it to finally finish it, and boy, was i in for a hell of a ride. Really, if you don't go in expecting much, and sit through the initial part, not to mention the dice roll mechanics which may put off some, you won't be disappointed.
Oblivion was just meh past graphics, boring everything. Haven't played skyrim.
edit- oh and Daggerfall had awfully long random dungeons so i don't think it's the best, though they were also VERY scary (which is great). I never finished it tho, but i don't think many would vote it for #1 TES.
Gamer Entitlement analyst, PhD. ..I edit every post.
Oblivion sucks outside the guild quests unless you mod it to s***, Skyrim is good vanilla, but is awesome with them.
But Morrowind is awesome vanilla (even on the xbox with that awesome crash when too much was going on like summoning and soul trapping all dem Golden Saints :3 and having to manage save files by deleting bodies and closing doors... 600 hours on that before I got the pc version haha), but becomes king of kings once you mod.
Maybe if Skywind is life itself like it looks, then I guess they will co-hold the title for me.
Morrowind, though like I always say, I do firmly believe that the game is 1000% better with the accurate attacks and readable signposts mods. Those are the two that I will always say are absolute must-haves for Morrowind.
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