Frap stuff is quite cool...very symbol-oriented layouts. The Instruo modules also have a really great post-Buchla semi-industrial look.
But if you want modules-as-art, I'd have to give the nod to Folktek. I have a Mescaline with the gold panels, and it looks at first glance like some alien artifact...but when you start to work with it, you discover how intuitive it all is. A brilliant balance between eye candy and pure function.