Nirn (as long as there isn't available all of Nirn in one game I'll be unsatisfied)
But that is unbelievable, so I think the best would be combination of Black Marsh and Morrowind
(and for those ignorant enough to claim that all TES games have only one province- newsflash- TES:Arena had all Tamriel, TES2 had Hammerfell and High Rock, TES3 had only half of province, TES4 was the only one that had one whole province (and sooo underscaled))
Really guys getting in between Dunmer and Argonians (and preferably totally killing both sides in Morrowind liberation process
) would be great