This is how I decide what to run:

68000:System 7.0

68020-68030-Low RAM 040: System 7.1.1 Pro

68040 high RAM-first and 2nd gen PPC-System 7.6.1

New World Macs-OS 9

Why do I skip 8? System 7.6 FLIES on my PPC Macs, and the Macs new enough that they shipped with 8 run just as well with OS 9.

8 is a transitional era, and short one at that. OS9 could had been OS 8.7, but Apple needed a new version for some reason.