Best GUI toolkit?

inb4 "embed a web server so you can use webpages!"

Constraints:
- free as in beer
- must be beautiful by default, no Tk shit
- is or can be made to interop with SBCL
- static linking
- at least works on Windows

What's the best automated testing and mockup framework for Scheme app development? I have ENTERPRISE ambitions, but the community doesn't seem to have done their job providing me with free shit very well in this case.