I still like the Archmage's chambers at the College of Winterhold, even if it lacks weapon racks and plaques. That mini tree garden with the light whisps floating around is just so cool looking. I think I'd add something like that in my own custom-made house.
Once the creation kit is out, I think I might try and make my own mini-mod involving treasure hunting like I did for Fallout 3, where you end up owning special weapons and a cool ass hideout. I remember writing traces of a character I came up with via notes. Every special weapon had a note next to it that described it and how it came to be, and I even made a pet mirelurk named Odysseus that you could have as a companion.
Anyway, the manor in Solitude is pretty nice too, even if the house in Riften has more weapon racks and whatnot. Riften is also a shitty town compared to Solitude, but I do like the area on the map that Riften is located. It has the least amount of snow I think.