I think the issue is more a case where Lloyd (the glasses character) and, especially, Keaton appear more "modern" to modern audiences. With much of his work coming in before the end of WWI, Chaplin comes off as much more Victorian (technically, I guess, post-Edwardian or early-Georgian) than those who hit their stride after him.
This is the critical difference..Chaplin was several years ahead of the other great comedians in his understanding of the potential of the medium, so like Griffith, there was a brief but important time when he was doing things both with performance and with directing and editing that no one else was doing and that made Chaplin the first movie superstar.
But-his great talent was in performance and editing, not in story structure. Even in his shorts, most of his great films are just great routines strung together. That problem was more obvious in his features.
So Chaplin made it much, much easier for everyone that came after. Lloyd and Keaton could absorb lessons by just buying a ticket, just like Chaplin absorbed Max Linder's ideas and greatly expanded on them. Lloyd and Keaton were much more into hanging gags on the clothes-line of a story, and thats something that Chaplin never really caught on to, he was more into making each scene work in the way he wanted. In other words, by the late 20s, other comics were zooming past Chaplin in the technical aspects of filmmaking, and it makes his films look old fashioned.
I think Chaplin is the greatest clown of the group in terms of performance and that makes him the most kid-friendly. Keaton has the best story structure and that makes his films, especially his features, the most fun to watch over and over. Lloyd, is well, the Third Genius. But here's my final point: Chaplin is the only one of the three who as a child went to bed hungry and sometimes didn't have a bed to go home to at all. This gives him a certain authenticity for the Tramp character-when he makes a joke about being hungry, I believe him-I'm looking at a face of a boy that had some real pain. That takes him into a social-condition kind of commentary that hungry people across the planet connected with then, and still do now.