Which fighter proved themselves against the most diverse range of GREAT fighters?

I agree with that although, the saying 'pure boxer' really suits him to a tee. Observers at the time mentioned that he was more 'American' in style than other British boxers, almost certainly due to his time in the US. He was more 'American' as he used inside fighting and boxed out of a crouch. It has been a while since I have watched footage of him, so can't quite make a proper judgement right now.

I was going to reply a lot sooner to your post above and mention Kid Gavilan but my computer was annoying me as I accidently kept deleting what I wrote.

Basilio IMO could be classed as a pressure-fighter as Teeto says, also from what I have seen of Williams he also applies pressure well. And with Turner you have a very gifted athletic fighter which must count for something. Also you could add Johnny Bratton to the list as 'boxer'.