Well, ASP has a nice one! I think the key to a UI is it needs to be customizable. Most people have their own way of working. Also, people have most likely been using another app at this point and are used to using it. Most basically copy Photoshop. In fact, I purchased Artrage, which has a unique UI and I proceeded to make it look as much like Photoshop as possible!
Rebelle has a nice interface. I like that you can make any brush the way you want on the fly. For instance, in Photoshop, if I want to blend or mix colors, I have to pick the Mixing Brush. I'd rather just change that kind of option on the fly with the brush I'm using.
I like the idea of a paint mixing palette (once color mixing is implemented). Photoshop had a plugin called Mixoos, but I couldn't really get it to work correctly. I think Painter has a separate painting palette.
Well, that's my 2 cents for now!
Bob