That’s the one! Usually it’s just the walrus line.. So appropriate for many situations. 
“I’ll teach your grandmother to suck eggs!”
Ask Dr Stupid!
It’s the Queen Bean!
That one gets quoted every time we have baked beans.
I’d have to go with
*“Happy happy joy joy.”
*
It’s my favorite quote and my favorite episode.
Trivial note: That and most of the other seemingly random spoken-word bits from “The Happy Happy Joy Joy” song are actual movie lines said by Burl Ives (the singer that Stinky Whizzlecheets is a parody of). The “I’ll teach your grandmother to suck eggs!” quote is from The Big Country.
Anyway:
“My collection of infectious diseases! VIOLATED!”*
*The way Ren says “VIOLATED” is what makes it funny.
“No Sir, I don’t like it. His butt’s too smooth.”
What about what Powdered Toast Man instructs the fellow he has saved, and tossed up onto his back as he (PTM) flies away: “Quick man! Cling tenaciously to my buttocks!!”