Eder Jofre & Juan Manuel Marquez are both among my four favorite fighters ever, so those are my two favorite old champs.
Luis Estaba. Made his professional debut at age 28. Won his first (and only) world title at age 37. Successfully defended it 11 times before losing it at age 39. Retired that same year because Venezuela didn't let people box professionally once they turned 40. Not a single one of his full fights is available, and I would love to be able to see just one. He passed away today. RIP to a man who had one of the strangest careers in boxing history and who had a lot of success despite the odds always being against it.
On that list Juan Manuel Marquez when he was fighting in the late 2000s I was a kid but I was fascinated by his style. He was one of the first elite Mexican boxers who I really followed.