I'll do five favorites and five least favorites.
Favorites on the Mac side
Mac OS 10.2 Jaguar (it made Mac OS X useable as a primary OS)
Mac OS 10.5 Leopard (most ambitious of the Mac OS X releases, added a ton of new end user features)
Mac OS 10.0 Cheetah (it redefined the modern operating system and saved Apple with its consumer-oriented UI, transparency, modern kernel, memory management, etc)
Favorites on the Microsoft side
Windows 95 (Microsoft's only major contribution to the desktop GUI, IMO; most innovative operating system Microsoft has ever released especially when you consider Microsoft's fast work on Internet Explorer including its crazy futuristic integration with the desktop GUI - WebDAV folders, RSS-like features, HTML customizations, etc)
http://en.wikipedia.org/wiki/Windows_Desktop_Update
Windows XP (NT kernel, modern search, after SP2 modern security, etc)
Least Favorites on the Mac side
Mac OS 9 (clearly a stop gap release with quirky features like VoicePrint password that never worked)
Mac OS 10.6 Snow Leopard (should have been a software update to Leopard users; still firmly believe the only reason it was released was because the majority of Apple's OS engineering resources were devoted solely to iOS during this period time)
[I won't even mention the System 7 updates that showed Apple's management had a complete lack of vision at that time]
Least Favorites on the Microsoft side
Windows ME (horrendous stability, rushed to market)
Windows 98/98SE (pointless upgrade although at least Windows Update was introduced with it)
Windows Vista/Longhorn (complete disaster all around, best example yet of Microsoft making billions of dollars on a clearly inferior product)