So starting the challenge at A, I'm going with an awesome movie by James Cameron, Oscar-nominated and groundbreaking for technology at the time of its release: The Abyss. I could also pick Avatar, but The Abyss came first so it has dibs.
The Abyss is a real genre-busting film. It has strong dramatic elements, but at it's heart it's great science fiction. But unlike a lot of sci-fi that takes place far in the future and/or in space, the Abyss is set in the modern world, which makes it more accessible to non-geeks.
Two parts of The Abyss stand out to me. First is the scene where Bud and Lindsay are trapped in a room of a research station at the bottom of the ocean, and the room is quickly flooding. They have only one dive suit with enough oxygen for one person to make the long swim to the main lab. The solution: Lindsay will drown herself and Bud, being the stronger swimmer, will tow her back. The cold water should help preserve her so they can resuscitate her in the lab. This is an intense scene, and the way the relationship between Bud and his estranged wife leads up to this pivotal moment earns high marks for drama.
The second stand-out moment is when Bud suits up to dive to the bottom of the deep-sea trench (e.g., The Abyss) to disarm a nuclear warhead. This is no ordinary dive suit: It's filled with breathable liquid. In order to survive the immense pressure, the dive suit cannot be filled with air. A super-oxygenated liquid is pumped into the suit, and Bud breathes this ("We all breathed liquid for nine months, your body will remember..."). The best science fiction is built on fact, and this liquid does exist, although it's not quite usable to the extent depicted in the film. I guess that's why James Cameron took a sub to the bottom of the Marianas Trench last week.
If you've never seen The Abyss, check it out. It's long, but it's worth it.