Hi Matt, thank you for your message my friend much appreciated!
Monzon doesn't look spectacular on film but he never lost to a world class opponent. He lost three in his first year as a pro and then went from 1964-1977 without a loss.
He had a somewhat listless style but he was sound defensively, had good power, great ring craft and a solid chin. He held the middleweight title for seven years before retiring as undisputed champion.
Monzon is one of the big four: Greb, Hagler Monzon and Robinson are four men, as Galactus says, that should be anybodies top four all time great middleweights, whatever order you have them in they should be the top four. All make a strong case for number one and all could hold wins over the others.
Hopkins in my opinion doesn't crack the top four, although he comes close, I rank him sixth with LaMotta at number five.
You could make a good case for Hopkins not having lost since Jones Jr in 1993 because every loss since then has been an SD.
He was one of the most dominant middleweight champions of all time, holding the title for nine years and making 20 successful defenses. He is also a great older fighter, still one of the world top light heavyweights in his forties. He too isn't always exciting but he has great ring craft and defense and is a master of controlling the pace of a fight. He is a tough fight for anybody because even if they beat him they don't look good doing it!
It may be a boring fight but it would be close and I would pick Monzon to edge the decision.