Really hard to say definitively, but if at gun point, you were to ask me to name my favorite Keaton short, it would be THE GOAT. Like THE GENERAL, it showcases one gigantic chase that takes up a good portion of the film that manages to be clever, funny, and tell a story all at the same time.
The surprise gags are awesome, and in case anybody who is reading has never seen this short, do not continue reading this paragraph. Like Paul said, the camera placement of some gags works wonders. It appears Keaton is hitched to a tire on back of a car to get away from the cops, but the car moves away with the tire still in place, the camera now revealing the tire placed on a stand separate from the car. The camera, as well as Keaton's worldview, gives an impression the tire is attached to the car, cleverly fooling the audience. Keaton getting past the train tracks egging the cops on knowing the train is coming? Brilliant. My favorite gag, though notvreally a camera trick, is after Keaton locks the cops in the back of the truck and gets away, a minute or so later, Keaton, resting on a sidewalk, has the truck randomly stop by him, dump out the cops, and the chase randomly resumes as if nothing happened. Again, brilliant.
Some great physical stuff here too. When the car drags Keaton across the sidewalk, the slide he takes that causes his body to contort in an awkward way had to be murder on the back. The fall he takes out of the hospital window is fantastic, but the greatest moment is Keaton, jumping on the back of Joe Roberts as a catapault to dive through the top opening of the door to escape the room. Very clever, funny, and an unbelievable physical act. No stunt doubles or camera tricks here. Watch the gag in the link below.https://me.me/i/buster-keaton-leaving-a-room-in-the-goat-1921-x-post-13054096
Also gotta mention the iconic shot of Keaton on front of the train, moving closer and closer to the camera until he's right in the audience's face, yet still refuses to break the fourth wall. We may be physically close to Keaton, but he's emotionally distant, as this is Keaton blatantly not acknowledging the audience, something he only did in that one banana gag in The High Sign, never to do again. Also, the complete antithesis of Chaplin, who made a career conversing with an audience. Two great clowns, two different viewpoints.
I absolutely love THE GOAT and can watch it over and over again. A definite favorite of mine.