honk
honk, honk
[Groucho]And two eggs![/Groucho]
I preferred his immortal:
Hoping this doesn’t doublepost…
whistles
[Chico]Doctor Hack-en-a-puss[/Chico]
whistles again
[Chico]He’s got a snake[/Chico]
Alternately, the entire “phone conversation” he has in Duck Soup. Excellent use of horns and facial expressions.
I’ve posted hundreds of great lines here on the Boards.
Do a search.
- Exapno Mapcase
(if you have to ask, you shouldn’t be in this thread)
Since he had only one line, it would have to be, “Ah-choo!” in At the Circus.
He may have also been part of the quartet singing “Sweet Adeline” from inside the herring barrells in Monkey Business. 
“You sure you can’t move?”
There, I got 'em all.
From his retirement party: “Now, as I was about to say in 1907…”
Does lipsynching count?
“If a mockingbird, could sing like you, it’d sing much sweeter than it do” (might not have the lyrics right)
It’s actually “If the nightingale could sing like you, they’d sing much sweeter than they do.” But that’s pretty much the same.
"Unaccustomed as I am to public speaking . . . "
– When he said he was asked to speak at a banquet or such (see “Harpo Speaks”). Brought down the house every time.
I liked his Maurice Chevalier impersonation.
He was a horny devil.
You know, he did write an entire book, Harpo Speaks. The cover drawing is a cartoon of him unzipping his mouth. I haven’t read it, but I’m sure it’s full of good lines.
To be technical - and realistic, Rowland Barber wrote Harpo Speaks, but I’m pretty sure that Harpo probably contributed something to it.