Probably Blackreach. It's horribly out of the way, and I hated doing the associated quests, but the location itself is incredible. Very reminiscent of Morrowind/Shivering Isles.
I'd have to say Blackreach as well. I want to build a little house in there and live there. I don't much like to be around people, neither does my character. Only problem is I think that big yellow orb is kind of ugly. :/
Its amazing, when you enter you find a skeleton dragging a cart with a chest with loot inside fo it, the place is beutiful as well. Its pretty out of hte way as well, other then the somewhat nearby nightgate inn.
There's some out of the way countryside with a crabbers shanty. I keep going back to visit the crabber. There's a huge waterfall, and I take a swim. It's very nice.
My favourite area right now has to be the open land between Whiterun and Rorikstead. It's so beautiful, and you can see what's coming so nothing can sneak up on you. Plus there's no blooming mountains to climb!
I never finished the game, so I didn't discover all places, but the most memorable for me were:
-Bloated Grotto (before and after some specific quest) -Valthume (loved the quest there) -first Dwemer ruin I found (it was the one west from Windhelm)
I just finished forsaken cave, theres a boss at then end and some neat loot at the end too, it turns out to be not as interesting as the beginning, still pretty neat. Named dragur, or maybe a dragon preist for higher levels.
I also loved bloated mans grotto as well. Kind of reminded of oblivion and morrowind, with the color blooms. And hob's fall cave is great. My favorite though would have to be the fallen log bridge over the waterfall near that guy's tomb. That was amazing.
I also just discovered this place, sorry for lots of comments, but alftand is epic too, its a tower, cool bridge, great view, and just nice design overall
I just found this lovely little grotto (there's a small camp of a poor doomed alchemist). Just a pretty little pool fed by a charming waterfall. Lots of nirnroots, butterflies, dartwings, and those little fishies. Just pretty! It's kinda near Falkreath, but I'm not remembering the name.