System 6 is an operating system that needs about 1 megabyte of hard disk space to be able to print, connect to a network, multitask, deliver wysiwyg, handle millions of colours, and carry a decent GUI, etc. etc. The only really interesting enhancements that were added to the later versions of the mac operating system were 32 bit addressing and lousy virtual memory management.
System 7.x, 8.x and 9.x are basically all the same stuff but more and more filled with bloatware and high level code to make sure that people would keep on buying new hardware because their old computers wouldn't run the new software fast enough. This is a process that began with the release of 7.0, an operating system that seriously slowed down a generation of macs that were running pretty snappy before.
Considering all this I think one can safely say that System 6 is the best pre OS X macintosh operating system ever written.