http://classicwb.abime.net/classicweb/advsp.htm
i'd also go with 3.1 and not 3.9 because 3.1 was the last real Commodore OS and has less bloat.
Actually, 3.1 was published by Village Tronic, not Commodore. So 3.0 would be the last "real" Commodore OS.
As for bloat, OS3.9 is obviously larger than older versions, but far from bloated. OS3.0 is bloated compared to eg. OS1.2, so by that logic OS1.2 or some similar vintage would be best.
