Hard to pick.
For sheer silliness, "Homer the Vigilante", driving home just how dumb Springfield's people are.
Jimbo: You let me down, man! Now I don't believe in nothin' no more! I'm goin' to law school!
Homer: NOOOOOOO!
Crowd: Flanders! Flanders!
Flanders: Well, I'm not really much of a leader.
Crowd: Someone else! Someone else!
Homer: I'm someone else!
Lenny: He's right!
Crowd: Homer! Homer!
For actual meaningful social commentary, "Seperate Vocations", perhaps better known as the Career Aptitude Test episode. Virtually every cartoon show involving kids going to school has copied its ridiculousness to an extent afterwards.
Planet Simpson (a book) declared "Last Exit to Springfield" (aka the 'Strike Episode') to be the best for various reasons.