I never had a lot of love for the System 7 style, but the Copland/MacOS 8 style was gorgeous
I always loved how Mac OS X Server 1.0 (aka Rhapsody) looked. I never got to use it, but it looked like Copland and NeXTStep had a kid (which is basically what it was).
I'm pretty sure at some point I used a Rhapsody Kaleidoscope theme on my PowerMac G4...