Charles in anyone's book should be considered top ten ATG. But Robinson was just a tad bit more consistent. Though Charles may have fought slightly better competition. But when ATG trainers , ATG fighters, and some of the best sports writers in history consistently consider Robinson the best, who are we, at least who am I to argue against that?
You could actually make a case for Charles. He went from being an amateur welterweight to becoming a professional heavyweight champion. No one has done anything remotely similar aside from Roy Jones and you could argue resume wise he's better than Jones too (Jones only had 1 hw fight and abandoned the weight class soon after while Charles defended his belt 8x). It's kind of apples and oranges since Robinson had way more depth and stuck around his 2 primary weight classes (welter and middle) longer than Charles did. Charles jumped all over the place but he was mostly prolific at light heavy. Robinson did attempt to conquer light heavy but couldn't quite manage it. This is a minor boost for Charles but not enough of an argument on it's own. Skill wise its also close. Charles is no slouch and was technically sound able to outbox you or have a phone booth fight. I think Robinson was a little more well rounded and polished while also having more p4p punching power and finishing ability.