Who is the best fighter that a 160 Ezzard Charles beat? Charles was pretty good at 160, but 175 is considered to be his prime.
He wasn't a world champion, but Charles was barely over 160lbs both times he beat Charley Burley, who is a top 10 ATG MW, imo.
Sky's the limit really. There's some guys who you'd have to make favourites based on either footage or prestige at the weight (Hagler, Monzon, Robi, Jones, etc) but I don't think it's unrealistic to say Charles could beat all of them. If pressed for a great middleweight who I'm happy picking to lose to Charles, I'd say LaMotta.
LaMotta was my 1st choice as well. I don't agree that RJ should necessarily be favored over Charles @ 160 though - even though RJ won a vacant piece of the title, he was no less green @ that weight than Charles was, & Charles' wins over a peaking Burley were no less impressive than Roy beating an up-&-coming Hopkins IMO.
I would clearly pick him to beat Hopkins. There is nothing Hopkins could do that Charles didn't already do at a higher level (except cry foul).