Well, I have found "Morrowind Comes Alive" to be really essential. It randomly generates NPCs all over the place every few days. So towns are a lot more populated, with interesting people like bounty hunters, merchants, trainers, and citizens. They change every few days too, so returning to town is always interesting.
As for graphics, the MGE changes everything for the better, allowing you to see faaar further. I also like "Vurt's groundcover" which creates grass meshes over the landscape, which makes the place a LOT more pleasing to the eye.