I'm not calling this list a "Top Five," because I trust you all to remind me of films I left off this list. I'm deliberately not including Godfather II, although it belongs on a Top Five list, because it's so obvious we'll just take it as read. But please, leave your own suggestions in the comments.
1. Aliens (1986). Fifty-seven years after the events of Alien (1979), Ellen Ripley is found floating in space in a cryogenic coma, the sole survivor of the Nostromo. No one believes her story, and she discovers that the planet LV-426, where her crewmates first encountered the alien, is now the site of a human colony. When the colony disappears, however, Ripley is the only person who understands what's really going on — and leads a team of space marines to stop the aliens once and for all. A lot of people consider Aliens better than its predecessor; I still prefer the terrifying paranoia of Alien, but Aliens is one kick-ass action movie.
2. Star Wars V: The Empire Strikes Back (1980) Way better than the original movie, if you ask me: better story and better acting (except for Billy Dee Williams, who looks great but seems excruciatingly self-conscious here). Luke Skywalker trains to be a Jedi under the master Yoda, while Han Solo and Princess Leia walk into a trap in the mining colony of Cloud City. Empire Strikes Back is a relationship movie, moving between the master-apprentice relationship of Luke and Yoda and the budding romance of Han and Leia to a final battle between father and son and the triumph of friendship above all. It's the only one of the "Star Wars" movies I'll watch any time it's on.
3. Ghostbusters II (1989). I'd be hard-pressed to choose between this sequel and the original movie. The sequel might get the edge, although it doesn't work completely if you haven't seen the first movie. Five years after the Ghostbusters saved New York in the first movie, they're still fighting lawsuits over collateral damage, and have been reduced to working as children's birthday clowns and TV psychics. After a series of strange things happen to Dana Barrett (Sigourney Weaver), she consults Ray and Egon, and the Ghostbusters are back in business. This time out, the enemies are the ghost of a Balkan tyrant and a river of pink slime that are threatening to take over New York. Bill Murray has proven himself a great dramatic actor, but his performance in this movie is as good as anything he's done, and Peter McNicol — as the tyrant Vigo's henchman — is brilliant.
4. Star Trek II: The Wrath of Khan (1982). I'm sorry, I find the first Star Trek movie (1979) almost unwatchable: ponderous, pompous, way too long. Wrath of Khan is superior in every way, including being more faithful to the spirit and content of the original TV series. Screenwriter Nicholas Meyer builds the story around a character from the classic episode "Space Seed": the charismatic megalomaniac Khan, returned from decades of exile and bent on revenge. As with most of the other films on this list, what makes this sequel so compelling is the focus on relationships over action — and if you can watch the final scene without weeping, I don't think we should be friends anymore.
5. Toy Story 2. A friend of mine had to take her four-year-old son out of the theater during a screening of this movie, because the scene where the toys cross the busy highway was too intense for him. Since jaywalking is one of my own phobias, I totally relate. Everything about this sequel is just a little more intense than the first movie; the stakes are higher, the feelings are sharper, the humor is more sophisticated. A toy collector steals Woody in order to sell him to a Japanese collector, and Buzz Lightyear must organize a rescue operation. Along the way Woody gets a love interest, the cowgirl Jessie (Joan Cusack), whose song "When She Loved Me" is just shattering.