This is a really hard question actually. My heart says Morrowind, but i have to be really fair here. Morrowind will always hold a very, very special place in my heart. I always liked fantasy and RPG's (mostly the standards: Baldur's Gate, Planescape Torment, Fallout 1 & 2) but i never experienced anything like Morrowind. Visited a friend′s house who had it and i was amazed. I also got it, installed it, failed miserably and didn′t touch it for quite a while. About half a year later i installed it again and it has been one of the most memorable and fascinating gaming experiences i′ve ever had. And Bloodmoon, and Tribunal, and then mods, owh my the mods.
I think my heart is biased because of nostalgia and impressions that were, at that time, completely new and groundbreaking. I didn′t have that same feeling of awe with Oblivion and Skyrim, however, when i look at it unbiased i think Skyrim is taking the cake. Ever since Morrowind i played each Elder Scrolls game as a Nord named Ragnar. Now in Skyrim, with dragons, cool quests, great visuals and sounds and a main quest wich i personally think is fantastic. Skyrim gets my vote. Thank you developers, for making TES in the first place