What is it? Mine would have to be:

Arnold: [looking for "rabies"in the encyclopedia, thinks he found it]Rabies, rabies. The leader of a Jewish community?!?!?!

Willis: That's rabbis you dummy!!!!

I guess it would be the episode where Willis' new band "The Afrodisiacs" hires Kimberly instead of Charlene as their female vocalist.

A band member states that both Willis and Kimberly are so talented, cause they sing well, and then asks Arnold what he can do.

Arnold's answer: "I eat!"

[This message has been edited by Ralph_Malph (edited 07-03-2001).]

I've got two favorites.
First when Willis is showing off his threads to his friends in the club meeting ep:
"When you've got it, flaunt it brothaa!"
-Willis

Second, in the ep Double Date, Willis is trying to find a date so he can double w/Arnold:
"I can't believe it, everyone is either busy or can't make it."
-Willis
(Kimberly walks in) "You can't find a date, the Billy D of Penthouse B!!"
-Kimberly laughing