I’m somewhat partial to the Telvanni Mushroom kingdom (the idea of, hey, here’s an acorn, go GROW your house) but Balmora has always held a special piece in my heart for being the first “big city” I’ve felt in a video game.
The transition to the Ashland and seeing a different biome entirely / grasslands / plains was also pretty incredible.
Ald’ruhn’s Capitol was also novel in design with the redundant rope bridges built on the inside of the shell of a gigantic upturned horseshoe crab.
Vivec’s cool but it’s only possible because of a demi-god’s literal meddling around with the terrain, and it’s too easy to get lost.
Caldera’s also nice, as well as Pelagiad.
I know I just named like ten places but Morrowind’s got a lot of diversity and biomes.