I never thought I'd say this about a silent film, but Harold Lloyd's Safety Last is one of the funniest movies I've ever seen. It's available from Netflix on disc 1 of the Harold Lloyd Comedy Collection. I saw a documentary on Harold Lloyd quite a few years ago, but it's only with the popularity of DVDs (and Netflix) that so many of these old movies are becoming so readily available. I also appreciate that this is a high-quality tranfer compared to the cheap-o editions of Charlie Chaplin and Laurel and Hardy I've seen. It does make some sense that the art of physical comedy would have been mastered during the silent era. Ask Father (which is also rolling-on-the-floor funny) only includes a few lines of dialogue.
Please note: Not all of his films (or all of the films on this particular DVD series) are suitable for children.