Foster is ranked higher at LHW than his resume indicates because of his ability there, and he was a non factor at HW. Charles's resume at LHW is light years better, as is his success at HW. Spinks had good success at HW, but not quite on the level of Charles resume wise I don't believe, and again, his resume is nowhere near Charles's. I rate Charles so highly based on resume. His resume is one of the top 5 best in the history of the sport.
Its one of those things, everyone has different opinions. You know its pretty crazy how someome can actually do a top 100 fighters of all-time list. And actually rate one fighter over another in every single position. Can people honestly look at the fighter in 40th position, and say the 5 or so below are better?. I don't think so, not with confidence. When people give their ATG lists, especially one thats so long, its like a raffle to a small degree in terms of where some fighters are placed. Obviously a fighter rated at 25 should be well ahead of 50th place in terms of their resume, coming off the floor, longevity, titles won, etc. But I feel its so hard to actually come to a positive conclusion that fighters in or around each other are put in their places with assurance from the person doing the ratings.
Het gets an automatic number one spot from so many, personally 1 - 4 resume wise is fine but i am talking more so reputation. I think there are a small handful that beat him but many people see the bad bald head and the Fu Manchu mo and get a little carried away IMO. Hell, many rating him unbeatable don't even know he was more a boxer type at his best, they see the Hearns performance and think he is some Mongolian War Lord. For clarity i rate him 2 or 3 ATG at 160 resume wise and about 4 or 5 head to head.
Obviously the difference is that Charles was a proven elite fighter in three different weightclasses, not just one, like Foster or two (with one of them being for only a handful of fights and not too impressive a run) like Spinks. Generally, fighters who only had top-flight success in one weightclass aren't ranked ahead of ones who had it in several, even if they are on even terms or ahead of that fighter in the given weightclass. Charles, while an immature young kid at 19-21, established himself as the #1 title contender at middleweight with wins over several of the best middleweights around, including all-time great Burley, but was shut out of the title picture and moved up to light heavyweight, where, after his military service and maturation into his mid-20s, he absolutely cleaned house and established one of the strongest resumes in history, making him the outstanding #1 contender, but was shut out of the title picture, leading him to move up to heavyweight, where he went through the ranks in decisive fashion and won the world championship, and continued to compete impressively at the elite level well into his 30s. Bob Foster may have been as impressive as Charles in the light heavyweight division, but since that was the only division in which he competed with any degree of world-class success, he doesn't touch Charles in a pound-for-pound sense.