Greatest? Humor isn't a competitive event, unlike Irish fiddling and pop folksinging. I will suggest, though, that Twain's piece on Jemes Fenimore cooper and the Skinner/Kimbrough book "Our hearts were young and Gay", as well as almost anything by Stephen Leacock managed to render me helpless with laughter.
    Leacock, who is largely forgotten today, was responsible for gms like "He jumped on his horse and rode off madly in all directions"