The Picard scene in "Family" was good (and Spock's death scene in TWOK would be right at the top of the list if it hadn't been nerfed by his resurrection in the next film), but it's this:
Use of Time wrote:
The scene in "Duet" where Marritza breaks down is pretty powerful.
... that's the undisputed champ for me.
"I am alive. I will always
be alive! It's Marritza
who's dead! Marritza, who was good for nothing but cowering under his bunk and weeping like a woman. Who every night covered his ears because he couldn't bear to hear the screaming
... for mercy... of the Bajorans..."
Still puts a lump in my throat even now. I think it's probably the single strongest dramatic moment I ever saw in a Trek show.