The ending when B-4 at first couldn't understand why his brother Data sacrificed himself to save Picard. And it seemed that all of Data's Tinkering, may have failed to change B4, until he starts singing the first line of Blue Skies. A song Data sang to Riker during the wedding reception.
The novelization of the film features a very bittersweet moment of heartwarming when Data dies. Just before he destroys the Thalaron Matrix, Data accesses every single memory he has of his friends on the Enterprise, and experiences them all as he's vaporized.
"You've earned a friend in the Romulan Empire. The first of many, I hope."
Worf, of all people, saying that "The Romulans... fought with honor."
Picard and Riker's final scene together:
Riker: Serving with you has been an honor. Picard: The honor was mine... Captain.