depends on what you're going to want to do with it. If you want one with relatively current-day functionality, you gotta find something that does at least 8.1. Honestly, 9.1 is really good. OS X defeats the purpose of being on a website devoted to classic macs

If you want something older and you're willing to work within the confines of the generation you choose, a late model 68k like the quadra towers or early early PPC machines with 7.6.1 or 8.1 is good. 7.1 is fun to play with, and it can surprise you with its versatility if you mate it with some later extensions and control panels from 7.5/7.6 and even 8.0... but you can only do so much with it.

You're really asking a vague question until you have some better idea of why you want it