Arguably means just that — you can argue that he is the best. It’s a matter of debate. Some say he is, others say he isn’t. Burt Boyar thought he was, and he was around ALL of them. So, he argues that Sammy was the best.
There you go!
Is there anything that's subjective in nature that isn't "arguable"?
To say that he's the best entertainer is "arguable" is to say nothing at all.
My opinion of him wasn't very high but the people who paid to see and hear him evidently had a better opinion of him and he was very succesful, that's not very "arguable".