As a general rule of thumb, any 601-based PowerMac I use OS8, 8.5/8.6 on 603-based machines and 9.1 on 604-machines... that being said I am running 9.2 on my 9600 with OS9helper. G3's and G4's from the graphite era, OS9.1/9.2 and OS 10.3 are where it is at... 10.3 actually runs faster than 10.2 or 10.1 on a given machine, whiilst 10.4 is a little sluggish on all G3's and G4's that are slower than about the 733 mark. The G5 I simply use 10.5 as it is the latest supported operating system and it runs flawlessly.
As for 68k machines, depending on variables... most 68000 machines I run 6 simply because it is small, fast, and doesnt chew RAM. I do use 7.1 on classics and SE's but most of them already had it when I got them and it runs reasonably well but RAM is an issue.
anything '020 or slower '030, I am most comfortable with 7.1 on them... on faster 030's I prefer 7.5 for it's added functionality and support, and I use this on most of my '040 machines as well. For what it's worth, I've never found myself with any desire to run 7.6, as it really seems it was designed more with PPC in mind but at the same time, PPC machines are a whole lot more versatile with 8-8.6.
As for 68k machines, depending on variables... most 68000 machines I run 6 simply because it is small, fast, and doesnt chew RAM. I do use 7.1 on classics and SE's but most of them already had it when I got them and it runs reasonably well but RAM is an issue.
anything '020 or slower '030, I am most comfortable with 7.1 on them... on faster 030's I prefer 7.5 for it's added functionality and support, and I use this on most of my '040 machines as well. For what it's worth, I've never found myself with any desire to run 7.6, as it really seems it was designed more with PPC in mind but at the same time, PPC machines are a whole lot more versatile with 8-8.6.