Buster Keaton’s was a great innovator in film. Â His Film Cops had the first use of a mobile camera (up to that point the camera was stationary and things moved around it). Â In The Playhouse, he comes up with one of the most spectacular special effects used to date: in the first part of the film, Keaton plays every part in the theatre, from all performers to audience members. Â There’s no CGI here, in order to come up with this effect, all parts of the camera lens, except where Keaton stood for this take were blocked, and the film shot, then the film was rolled back, and keaton moved to his new position (and if needed, new costume) and the film was re-exposed with the other spaces blocked. Â Time consuming, yes, but the gag is most definitely worth it.
The short, Cops contains the afore mentioned use of the moving camera.  It also displays Keaton’s acrobatic ability, which alone is worth the watch.
Chaplin’s The Pawnshop is an excellent example of the slapstick for which he was known. Also worth noting is how the Tramp goes from playful mischief to being ready to throw down in a flash, and how somehow the audience never loses the empathy they have with the character.