Worf disobeying a direct order from his captain and NOT singing when Picard instructs him to in Insurrection.
"Sing Worf SING!"
*Stern shake of the head*
The line about misunderstanding what Picard meant about "knowing Gilbert and Sullivan" was quite funny too, was nice for Worf to have a funny scene in that film before he got lumbered with all that Klingon acne
Also: Nog's dancing at Jadzia's hen party.
Picard pulling that big gurning smile while walking into the Turbolift in series 1.