For me, the high water mark is Snow Leopard. I don't think I remember any single operating system iteration for which I held such little contempt. I only have fond memories of it.
(Second place is probably a tie between Windows 2000 and Windows 7 depending on how nostalgic I'm feeling.)
I was perfectly happy with the default/native skin for Windows 7, either with or without its transparency effects.
Like most people I greatly preferred Windows XP in its classic mode because XP's default "luna" skin was so hideous. Whereas the Windows 7 classic mode felt slightly broken and abandoned—new UX behaviours didn't quite line up with expectations set by the traditional interface design. A change of appearance is appropriate when there is a change of behaviour.
(Second place is probably a tie between Windows 2000 and Windows 7 depending on how nostalgic I'm feeling.)