The "big four’ was a creation of James Agee in an article in Life Magazine in the 40s, when memories of the films were weaker (with no TV, most people hadn’t seen any of them in years) and only Chaplin was working (and then sporadically). Some have said Agee missed other, better commedians of the time.
I’ve seen all four. Langdon is generally the weakest – a one-note character (Agree pretty much admitted as such), who was much more dependant on his writers and directors than the others (Frank Capra directed his best films, BTW, so he may be more a Capra creation than a character in his own right). Once he started directing himself, his career fell apart, and indication he shouldn’t be listed as highly as the other three.
Lloyd was a great gagman and superb athlete, but his persona is very much of the 20s, and dates badly. In addition, he didn’t really try to be more than a gagman, which puts him behind Keaton and Chaplin.
Keaton’s by far the most modern of the four; indeed, he had a critical rebirth in the 60s-70s (See his appearance in “Sunset Boulevard”: intended to show a bunch of wash-up has-beens). His gags were more modern, more elaborate, than anyone else. But he was also a master of the small gag (the funniest thing about the train falling into the river in “The General” is the shot immediately afterward). His sentimentality is less obvious than Chaplin’s.
However, Chaplin gets my nod if I’d have to choose the best. Chaplin’s persona is timeless, his gags – big and small – brilliantly imaginative. But Chaplin did introduce depth and pathos into his comedy, making it all the richer. The final shot of “City Lights” is incredible – joy, sadness, and ambivalence all mixed into a heartbreaking moment. Chapin went for much more than comedy, and that gives him an edge to Keaton. He wasn’t just hilarious – he was so much more than comedy.