Data when his emotion chip starts to overload. Especially since he's still laughing while he's malfunctioning.
Data: (laughing his butt off at nothing) Geordi: Data, we don't have time for this! Data: (laughing) I cannot help myself! (still laughing) I think something is wrong! (continues laughing, then gasps and staggers forward as if in pain, then resumes laughing) Geordi: Data? Data: (strained laughter accompanied by pained gasps, then suddenly falls over, his face blank)
Followed shortly by Data showing genuine fear when Soran holds him at phaser-point.
When you consider that Data can think about a dozen different things at the same time - go back to every episode where someone in Data's position would have had a significant emotional event (like watching his daughter die). How many moments of anger/laughter/anguish was he having at the same time?
The crash scene. Most of the bridge crew are downright terrified as they're about to slam headfirst into a planet. The movie pulls no punches either, giving us the logical conclusion of bringing children into space, as the audience gets treated to an entire extended sequence of children being evacuated and then screaming for their lives. Data sums it up nicely:
Fortunately, Picard and Kirk undo it, but the moment that the Veridian sun is destroyed must be this for everyone on the Enterprise. They barely survived the saucer crash, the ship is in shambles, there is no way for them to get off the planet without someone else coming along and giving them a lift... And then the sun goes dark. Not "night falls," but the sun is abruptly gone. And then the shockwave hits, shattering the planet and everything on it.
Look closely at the crashed saucer section before it's consumed by the shockwave — you can just barely make out the survivors vainly trying to run away from the oncoming wave of destruction. And when you consider that the shockwave appears to behave somewhat akin to a concussive blast rather than a fireball, instead of just being burned to ashes in a nanosecond, this means that all 1,000+ people on the ship — including civilians and children — were ripped to shreds and turned into Pink Mist.
And then you remember that, while Veridian III was uninhabited (not counting the crash survivors), Veridian IV had a pre-industrial civilization on it. All that life, lost. Because one man couldn't accept the death of his family.
