(Warning: long rant, but you did ask, so you shall receive ... ;-) Some people just never learn)
My favourite was System 7, because it was still a simple and honest system, before it suffered the inevitable overcomplexity of any modern OS.
A random list of things I like about Mac OS (working or broken, present or absent from Mac OS X, Linux and Windows), bearing in mind I've barely used it in two years: type and creator codes and file type autoregistration (BNDL/FREF) [4], the extension and URL mapping database [1], Apple Events [2] and AppleScript [3], the custom icon facility and icon labelling, the global menu bar, the Control Strip, the global context menu system, the way that both programs and system extensions are packaged inside single files (even my NIC driver is one single file with an appropriate name
and icon, instead of heaps of unreco~1.ble files), the ability to switch keyboard layout across the whole system without programs vetoing or ignoring the change and the layout flapping back and forth as you switch application, the volume-centric storage model, being able to rename files that are open and watch the program that opened them update accordingly, being told by the OS exactly which program has locked a file on a volume you're trying to eject or that you're trying to empty from the trash (you can trash files that are in use, just not empty the trash), having a Temporary Items folder that is automatically emptied during boot so you don't ever get gigabytes of garbage build up in there ...
No system is wholly good or wholly bad, and what pains me about Mac OS 9 is that nerds slam it for having no memory protection, and no command line, and treat it like it's a leper that also has the plague. Windows people get put off by insufferable Mac devotees (thankfully I grew out of that phase!) Every other innovation and feature is conveniently forgotten by people who just won't give it a chance.
Hell, I gave Linux a chance, but it let me down
I kept flogging its dead horse until my memory upgrade in my StarMax 4000/200 caused it to run like it had no CPU cache, and I decided I couldn't take it any more. I was sick of every last single thing crashing, breaking, and self-destructing. I use Linux (Debian and Ubuntu) for servers at work, command-line only (most don't have X11 installed at all) and they're enough of a headache
There are a lot of systems that we can still learn from, Mac OS just happens to have scraped on by. AmigaOS as I understand it had some great innovations, and
RISC OS did also.
[1] You'll appreciate it a lot more when you look up Simple MAPI and see how that works
[2] Compare this with the mess that is Windows's various attempts at being equivalent; D-Bus seems to have promise. Also, on a Mac, you don't get this nonsense of opening several files at once and having the shell spawning one process for every file, which may co-operate and marshall their efforts, get confused (Winamp), or just explode (IcoFX). Application capability and requirement manifests (like the Mac's SIZE resource) could help here by instructing the shell how to handle multiple files as one of the flags.
[3] AppleScript would have single-handedly prevented macro viruses because utility scripts live outside of documents, but Microsoft had to refuse to learn from Apple's success (I think WordBasic may have come first in MS-DOS but the mind-blowing mess that is VBA, especially in Outlook, could have been completely avoided, and Outlook could be as scriptable in Windows today as Outlook Express was on the Mac)
[4] I love how Windows suggests Photoshop to open files that Photoshop can't open, to which Photoshop turns around and tells you to get lost, because Windows doesn't have any way to understand what any particular application is capable of. It has very primitive file type mapping but that relies on programs not treading all over each other!
