MacOS 9 would have to be my favorite, though System 7 is good too. It's one of the few where you can tell they hired real graphic designers to do it up. Something went wrong once NeXT took over at Apple. Rhapsody, NeXT with OS9 window borders looked nice enough. Aqua forward is a real let down with the ugliest window scaling gadget ever made, which doesn't fit the rest of the window look. There are still pure NeXT bits still in there but the Aqua to brushed steel to whatever it is now has never really gel'd or ultimately felt like a cohesive design. It also, even more than modern Windows, wastes vast quantities of pixels on nothing but blank space. They're both the equivalent to those phones with giant number you buy for old people. Still, I like it better than Windows.
(Rhapsody, best of both worlds MacOS + NeXT)
Most Amiga interfaces are hampered by non-uniform scaling issues from wonky, legacy pixel aspect ratios. MorphOS is pretty nice looking and maybe one of the prettiest bitmap interfaces which gives it a throwback quality despite being a modern OS.
Most UNIX UIs, besides maybe Open Look, are just hideous and not terribly useful in any case, post-NEWS IRIX being one of the ugliest and clunkiest ever (NEWS, on pre-Indigo Irises was actually quite nice but the move to an X-Windows based UI killed performance while getting ugly at the same time) since Motif was just awful. UNIX/Linix desktop environments are usually ignored as desktops anyhow and mostly you're determining what sort of bars and gadgets you're going to have on your shell windows.
