If we include Ring belts Hops would be ahead of everybody else with 15! In order to prevent endless discussions about which world titles are "real", and which are not, I always add the "recognized by BoxRec" bit for these types of questions. This doesn't necessarily mean, that I believe BoxRec is right about everything... but it ensures, that we are all on the same page.
Yes, it was Luis Estaba. I got home late last night and when I saw the question, I thought it was one of the real lighter weights, but couldn't remember the name. When I did some research several years ago I was surprised to find out that when he turned 40, he had to retired according to Venezuelan law (August 13 1978 Estaba turns 40 and has to retire according to Venezuelan law). I didn't know if that was the answer but I knew he had to be close. I finally found his name and checked it out, but it was too late. Great Question, you can win a lot of bets with this one.
When I posted this question, I kind of expected people to go with either Moore or Hopkins... which of course was what happened. I couldn't use the thing about Estaba being forced to retire at 40 as a clue... because that would be the same as handing you the answer on a platter. That is exactly the kind of thing you would know by heart - as I believe, you remember just about everything you have ever read! Am I right?