My vote would have to be the scene in "Mrs. Wrong" where Steven begins singing Swing Low, Sweet Chariot", pauses, and then Alex and Elyse join in. You could tell that Michael J. Fox was about to bust trying to hold in his laughter. That entire two-part episode is filled with great scenes.

I always love it when a Keaton breaks out in song! They are so good at it--Steven and Elyse especially.