I only have been in Blackreach briefly, but felt amazed... totally amazed. I found it scary to be honest. Didn't feel comfortable at all and I tried avoiding those chaurus-beasts to all costs

After locating the elder scroll I got out as soon as possible. With my second character, I am about to do that part of the MQ again. Will spend more time there now, it's worth it.
Other places in the game that impress me are Labyrinthian, and some ruins/temples have an unusual design. I forget about names, but one in the southwest (southwest from Falkreath if I am correct) gave me the feeling I was in an Indiana Jones/Tombraider-setting. Really cool.
The game does have some memorable outside locations if you have an eye for it. Wandering in thick snow west from Winterhold is very special, if you barely can see a thing and walk alone between walls of ice and snow.