I think System 7.6.1 has to be my favorite older version of Mac OS. Of course, that was the version on the Macs when I was in design school so while I'm aware of System 6 and before I had no practical experience with it. I also skipped iOS 8 entirely and got my first job on Macs running iOS 7 and later iOS 9.
Graphically I liked it a lot better than anything until OS X 10.3.
I think 7.6.1 was also amongst the most stable of classic MacOS releases too. It's also not greedy on system resources.
I've standardized on 9.2.2 on my G3 PowerBooks as I've replaced the drives with solid state alternatives, but have yet to do the same with the earlier models. I'm still trying to get a SCSI2SD 5.5 to work that I can use as a boot drive/MacOS installer on the 100 and 500 series PBs - otherwise they're running the various MacOS versions from when I bought each one.
I did rather like the simplicity of System 6, but it was rather basic! Back in those days, the OS was mostly there to get the system out of your way as a user - System 6 did that pretty well!