I own the house in Riften, and like it, but is there a better one? (I have about 20,000 gold)

For the "bare" minimum you'll need 25,000 for Solitude...so that is out. I hear the Markarth one (Vlindrel Hall) is really nice and thats what I'm working for now.

If you want for pure storage purposes and ease of access Whiterun is your choice. It also at the center of Skyrim which is big plus.