Back in the 70s castle films advertised within every boxing magazine. You could buy all these old bouts in 8 mm form for 10 dollars a pop. I owned dozens of these. Louis had the largest inventory of bouts.
I remember going to the library with my mom when I was real young and they had some 8 mm films ... including the Tunney-Dempsey fights (one one reel, I think). We had a projector and so checked them out. I was fascinated.
Which fighters were the most filmed and readily available to view? Louis, Robinson, and Ali. Initially, I read the question as an inquiry as to personal fight libraries a fighter would keep for study purposes. To which I would think the answer is that gyms and trainers would be more likely to have the extensive libraries.
Before YouTube? YouTube launched in 2005. There were tons of fights available before then, particularly around the start of the 1990s. I started watching boxing in 1975 (when I was 10), but I didn't start recording boxing matches in 1987. That's when I got my first VCR. At the time, I recorded everything I could. In the early 90s, I began posting ads in the back of boxing magazines where other people recording fights were trading them. I wish I still had some of their lists. It seemed a lot of people began recording fights off television in the mid 80s, when VCR prices came down quite a bit. Occasionally, we'd find a guy who had been recording fights on Beta tapes since the late 70s. Those guys were always treasured because the quality of their bouts were good and they hadn't been traded all around. There weren't many. I remember as far back as 1980 or so seeing Kurt Noltimier's name in boxing mags looking for trading partners. But he had some weird practice at the time where he only recorded and wanted fights from bell to bell. he didn't want intros, prefight or postfight, because it was a waste of tape. (I guess he'd started in the BETA days.) But you could chart the big jumps in what fights were available with the changes in technology. 50s and 60s - There were guys who recorded fights off their television screens using film cameras in the 50s and 60s. There wasn't a lot of trading between them. People tended to buy original films from a guy's relatives after he died - because you had to take your film to someone to get copies made. You couldn't really do it yourself at home. And it was expensive. 1970s BETA recorders - When video recorders became popular, at first BETA was the preferred tapes. They were bigger than VHS tapes but the picture quality was better. But Beta tapes were expensive (like $25 each) and originally you couldn't record over them. You also couldn't watch what you recorded while you rewinded and hit pause. It was very clumsy. So people were picky about what fights they recorded, because you usually had to keep the commercials in and it cost a decent chunk of change (for the time) just to record one 15-round fight. 1980s - When VHS took the market from Beta tapes in the 80s ... a lot more fights became available and a lot more people began trading with each other. Again, these tended to be just fights other guys around the country or the world happened to catch recently. Not a lot of older fights were available. The Ali, Louis and Robinson fights from decades earlier that were being traded around tended to be copies of copies of copies of copies ... so the quality was usually pretty awful. ESPN had a show called SUPERBOUTS, which televised some of the fights from Bill Cayton's library. But they tended to show the same dozen or so fights on a loop. The first Ali fights I watched on VHS (that I received in a trade) that weren't shown on SUPERBOUTS were Ali-Wepner, Ali-Lyle and Ali-Patterson 2. I hadn't seen Ali-Lyle since I watched it live as a kid, and I'd never seen Ali-Patterson 2. (I don't think it had even been rebroadcast since it aired in 1972.) And the quality was almost unwatchable. But I loved it. Because that's all there was. It was also difficult for U.S. fans to trade with fans in Europe because the U.S. machines recorded on NTSC format and the Europeans recorded on PAL. If you received a PAL tape from Europe and played it, the image was scrambled -- and vice versa. So I remember having to go to places like Radio Shack and paying $25 a VHS tape to have them convert PAL tapes I'd received in trades to NTSC so I could watch them. Again ... it wasn't easy. And it was expensive if you lived in the U.S. and wanted to watch fighters from England or Australia I also had to subscribe to UK boxing magazines just so I knew who the hell some of the guys I was watching were. There wasn't any way you could read what fights were taking place this week or next week in another part of the world until they had already happened and were covered in a boxing magazine a couple months later. 1998 - ESPN launched ESPN Classic and began televising boxing matches from Bill Cayton's library from the 50s and 60s and even more from the 1970s. That's when the Ali and Robinson fights started to look like the other fights we'd record on our VCRs. ESPN Classic wasn't available all over the U.S., so any traders who had it suddenly were in demand. Late 1990s - The internet really took off. But it was nearly impossible to watch video with a dial-up connection. And there really wasn't any video to watch. This is also when more guys started reaching out to contacts at television networks and fights from the network master recordings began to appear. This was also a HUGE deal. Suddenly, the ESPN classic fights weren't as important as getting an original copy with the original announcers that didn't have ESPN logos all over it. 2000s - DVD Burners became available. I think I bought my first one in 2002. It was incredible. Shortly after, Torrent sites began to pop up. So guys with DVD burners could copy their VHS tapes onto DVDs and convert them to smaller files and upload them to torrent sites. When these were first discovered, it was a godsend. Late 2000s -- People began uploading the fights they'd been recording forever and transferred to DVD onto sites like YouTube. So ... consider yourselves lucky. You now know who is fighting anywhere in the world before it happens ... and you can usually find a stream of it somewhere and watch it live ... or you can download it from YouTube or a Torrent site just after it airs. THIS is the golden age for boxing fans. It's never been easier to watch boxing than it is now. Never.
In answer to the original question, though, I'd say any fighter who boxed as an amateur in the mid to late 80s ... like Lennox Lewis or Roy Jones (let's say) ... who then turned pro in the late 80s and fought thru the 90s ... you could readily find more on them (amateur and pro) ... than guys who came before them. For guys who were amateurs in the 70s and turned pro on the 70s (and earlier), it's more difficult to find EVERYTHING they did, because there were fewer people with home recording devices and there were fewer people sharing what they did have. And when those people who did record things but didn't share died, a lot of their stuff wasn't kept by family members.
Does anyone know what happened to the collection of Jimmy Jacobs and Bill Cayton after those guys died? They were supposed to have had the world's largest fight film collection.