Just some comments on some earlier posts:

I've recently rediscovered Opera 5 for Mac. Much faster than any other browser on early PPCs. Way faster than netscape, faster than IE, faster than Opera 6.

Carbon apps are reeeeaalllyyy sssllllooowww on OS 9, better to get the Classic OS native version if it's available. (if it has that funky OS X alias and hidden folder setup, it's usually carbon).

If you want to run the latest and greatest, you need the latest and greatest hardware.

Older hardware has an optimal operating system which is not necessarily the latest system that will run on it. As a general rule it is the version pervious to the last system that will run on it.

My rules are:

68000 - System 6

68030 - System 6, System 7.1.2P, System 7.5.5 (for special applications/reasons)

68040 - System 7.1.2P, System 7.5.5/7.6.1 (for special applications/reasons)

Nubus PPC - Mac OS 8.1/8.6

PCI Power Mac - 8.6 (mainly because of USB support), 9.22 (if it's a faster PCI Mac.

Any Mac with a G3 (native or accelerated) - 8.6, 9.1, 9.22

G3 Beige - 9.22, 10.2/10.3