Blackreach is absolutely jaw-dropping; I never expected anything like that. I still need to go back and explore the various sections -- I only realised it was so huge when I checked its wiki page after I completed the quest involved there.
Other than that, I love the entirety of The Rift; its tranquil feel evokes a sense of serenity I don't think I've ever felt in a game. Not in a long time, anyway.
Spoiler And now that it's Imperial territory, I'm even considering buying property in Riften, even it is, quite honestly, a crime-ridden cesspool. Just bought Proudspire Manor, though, so I'll need to jazz up that place first. Beautiful home in a beautiful city.
Skyrim's art design is sublime and I haven't explored all of the map yet. I'm yet to visit Markath and by virtue of that, explore the surrounding regions.