The "conventionally attractive" Marx brother, who usually played the straight man. Still better known than Gummo, if I were to guess.
We're personally fans of Harpo, mainly because of the metaphorical whiplash that comes from watching his scenes. One minute you're laughing at the comical clown or admiring his musical skill, and the next you realize that he's actually a dangerous maniac. (Run, children!!!! He is not safe!!)