As for Trek moments, I would nominate Spock not letting Kirk save Edith Keeler's life in City on the Edge of Forever.
As it turned out, it was actually Kirk that kept McCoy from saving her. A great episode with no tidy solution. It's also peculiar in the fact that Kirk utters, "Let's get the Hell out of here" before they transport off the planet. That language was not common on television at the time.
Here's my similar list of all time biggies (in no particular order):
City on the Edge of Forever
SOYLENT GREEN IS PEOPLE!
2001 is made; turns sci-fi into art.
The opening shot of Star Wars. When that Stardestroyer took 15 seconds to pass into the screen, the audience roared with approval. It was the cinematic equivalent of Jerry McGuire's "You had me at hello."
Planet of the Apes ending (or the whole damn movie. I love it. The remake is an abomination)
Alien - yep, the Chest Burster.
Babylon 5 completes 5 year milestone.
Twilight Zone - To Serve Man (and several others, but that one is just such a hoot)
The Matrix - Combine science fiction and Chinese action flicks in medium saucepan. Stir in groundbreaking cinematic effects. Let simmer 2 hours. Enjoy!
Silent Running - Douglas Trumball's cautionary ecological tale featuring the Valley Forge, one of a few ships carrying the last of the Earth's forests. NOTE: film stock of the ships was used in 1978's Battlestar Galactica and the ships were recreated again for the new Galactica.
Dark Star - the low-budget, hippie-surfer-astronaut flick that gave us the beach ball monster, the talking bomb and the frozen captain. Started the careers of Dan O'Bannon and John Carpenter.