I'd have to say that my favorites are Mac OS 9.2 and Windows 98SE. They're good at letting you get work done without being too overblown or hard to understand. Runners up include BeOS/Haiku, AROS, and OS2/eComStation.
Everything else is either too simplistic (DOS), too overwrought and slow (Windows 7), or breaks compatibility at the drop of a hat (desktop Linux in general, Mac OS X)