I believe Ray Arcel holds the record for most Champions trained at 20. Therefore, he is tops of all time.
Arcel was brought in for that fight specifically to help Futch come up with the gameplan for Cooney. Despite his comments in public Holmes took Cooney very serious. I think Arcel was in Homes corner for one or two more fights after that. His comments in the corner between rounds in the Cooney fight were hilarious. "Don't let this bum take your title"