It's a close tie between "Krusty Gets Busted" and "The Crepes of Wrath".
"Busted" had some great direction by Brad Bird, arguably the best of the first season. It also had arguably the best script, too. "Crepes" crept towards the type of sarcastic edge that The Simpsons would more fully develop in season 2, and had a good mixture of funny moments and serious ones, like when Bart was suffering with the two French jerks.
"Crepes" also has the recognition of one of the funniest season 1 lines, upon an overworked, disheveled Bart eating a single grape: "Ungrateful swine! We give you food, we give you shelter, and THIS is how you repay us?!"
Least favorite is definitely "Moaning Lisa". I appreciate that they developed Lisa's character in it, but it's just not very funny overall, outside of a couple moments during the Bart vs. Homer boxing video game (I always crack up at Homer waking up from his nightmare screaming, then calmly and immediately going back to sleep).