4.3 BSD was the best of its time.
Oh, hail yeah!
Amiga OS comes second. A multitasking OS that can run in only 256K of RAM.Best OS of the 80's and early 90's.
If you're talking Windows, Windows 2000 Professional and Windows 7 are considered by many the best versions of Windows produced by Microsoft.
Mind you, I think Windows 7 really lucked out because at the time of Windows 7's release in October 2009, computers with x86-64 instruction set support were widely available and very reasonably priced. As such, it got adopted with lightning fast speed.
OSX: probably about 10.4
Windows: Windows 2000 or Windows 7. Both seem stable and lacking in some of the garish rubbish you get on the other versions.
Unix: Possibly BSD 4.3. Certainly contained a lot of important stuff
Honourable mentions: Risc OS (booted from ROM instantly) and AmigaOS.