My favorite TES game is Morrowind (even though it's the only one I haven't finished!) because of it's depth of story and wide range of skills. I also like it because of some of the amazing modding projects. My favorite Morrowind mods are:
Tamriel Rebuilt. This is a mod that aims to create the mainland of Morrowind in the same way that Bethesda would've if they had included it in the vanilla game. The latest version at time of writing is "Sacred East", which is the third out of eight planned developments. I would highly recommend this mod for all Morrowind players. When installing, make sure to manually change the [Archives] order in the Morrowind.ini to:
Morrowind Overhaul. Not really an individual mod, but it is a collection of many mods that, when combined, make Morrowind look about the same as, if not a bit better than, Oblivion. Also highly recommended, and is probably one of the easiest mods to install.