Wow! It's hard to pick just one!
But I will say the one where Ernie is painting a picture of Bert. Ernie won't let Bert see it til he is completely finished. Then, when he is done painting, Bert is so excited to see how his portrait turned out. But the painting has Bert wearing a beret (I think), glasses, and a beard. Bert gives the classic "ER-nie!" line and says how that looks nothing like him. Ernie says, no problem, I can fix that. So he slaps a hat, glasses, and fake beard on Bert to make Bert look like the painting. And of course, Bert gets mad.
I liked it when I was little because it was hilarious (and still is). I like it now because of the way Ernie handles the situation. He never lets anything get to him.