Had to be the little game that could I'm afraid. So many hours, so many moments, so many memories and so many dreams tied up in Morrowind I cannot separate the music.
Nice point...
I kinda like how, even though it was probably unintentional, all 3 themes tell a sort of story when put together, like the 3 acts of a play.
The Morrowind theme is a beautiful, calm little piece that reminds you of dawn and sets an upbeat mood. It's sort of an introduction to the Elder Scrolls (I know Morrowind was definitely not the first TES game, but III, IV and V just sorta fit together nicely). Then Oblivion's theme is the beginning of the main conflict, with a more "the end is nigh" sound. And finally the Skyrim the final act where the hero rises to the occasion and battles to the end in the face of defeat, with epic chants that remind you of an army assembling for one final push.
I ain't saying the games are like this, but the themes sort of are.
A more curious point. Of the three, the only one I cannot chant 'Morrowind' to is Morrowind, heh.
Morrowind with lyrics... http://www.youtube.com/watch?v=HKXn_BcDqbo