Arcwind point and Bard's leap summit are both spectacular, while the Forgotten vale is magnificently haunting. However, I'd have to go for Blackreach, but to explain why will take a moment, so get comfy.
I played a thief on my first playthrough, which meant that dungeons were slow. They consisted of default crouching and liberal use of Aura whisper (I don't have detect life because I'm not a mage). Furthermore, since my most potent weapon is a dagger I am also very careful when engaging an enemy. Essentially, this makes even ordinary dungeons a lengthy affair.
Blackreach, however, is not reached through just a standard dungeon. I didn't travel here until Elder Knowledge, so I had to pass through Aftland, which is a particularly long dungeon. When you also consider that I travel extremely slowly while carefully picking off enemies this made for a staggeringly stressful journey filled with potion-chugging when I found myself spotted by hordes of Falmer, as well as with a permanently-heightened concentration.
This was incredibly mentally draining. Being THAT focused on something for that long while journeying ever deeper into the earth, all the while knowing that the only (known) exit was being left increasingly far behind just made the intensity and claustrophobia all the more unbearable.
The feeling of slogging through all that for so long to emerge into a colossal underground cavern filled with what is best described as "drugs" was stunning. The sheer vastness of this space coupled with the uniqueness of the city itself were an amazing way to top off the most mentally-difficult dungeon in the game for a character that is so dedicated to stealth and cunning.
There are plenty of remarkable moments in Skyrim - unfinished as it is - such as the city battles in the civil war quest, or the ascending of the Throat of the World, or the opening attack scene, but the trip into deepest, darkest Blackreach was as good as anything I've played in a looooong time.
Skyrim at its absolute best.