I don't know the answer to this myself, but it would be an interesting statistical question for who is/was at the top and why? I know during Joe Louis' reign that Arthur Donovan was the main ref for most of his title fights. A side-note is that this was one of the controversies of Jersey Joe Walcoott's camp before their first fight, December 5, 1947. His management petitioned the New York State Athletic Commission to use a ref other than Donovan, and they got Ruby Goldstein, another top ref at the time. In fact, it was Goldstein who saw the fight for Walcott, while the other two judges gave the fight to Lewis. Most believed that Walcott should have won.
You had other New York refs at the time because from about the 1940's-1970's New York was the star attraction for big time boxing: Eddie Joseph, Arthur Merchant, Tony Perez, Harold Valan. Than you had some well noted oversees refs like Stan Christodoulou, and Waldemar Schmidt. I think Mills Lane became a ref in the early 60's.
So who is the top ref for most fights worked and if different who is the top ref for most world title fights worked? I will consider a World Championship one or more of the four alphabet belts on the line for a title, or the Ring Title. But in my view after about 1990, the Ring titles became more diluted than they were up until the late 70's.