Morrowind is so cool looking, so original and alien. It's amazing! I have few words to describe it accurately.
Skyrim I love, especially the mountainous regions in The Reach, or the snowy areas further north. Hjaalmarch and Eastmarch are great sights as well.
And I really want to know more about the inner regions of Black Marsh, a mostly untouched swamp inhabited by lizardmen, I really want to know what Bethesda would have it look like.
Morrowind by a country mile. I love the Near East influence from Assyrian, Babylonian and Semitic cultures that inform the Dumer nation and the sheer alienness of the landscape with it's giant mushrooms, salt swamps, ash deserts and volcanic regions as well as the crazy lizard\insect\crustacean animals that populate it. Also I really like the post-apocalyptic fantasy feel that it has.
I will always like High Rock more because a large portion of it is fertile with temperate weather, providing the body with many sources of food. The forests of Glenumbra and the rolling hills of Stormhaven are nice too. It is across from Hammerfell so that is a plus.
The Iliac Bay is boss.