Here are my nominations (chronologically listed, not ranked) with brief comments inspired by
@epenguin.
I also add below some honorable mentions which could have been on a larger list of more than 20 movies.
Science Fiction:
1. The Empire Strikes Back (Star Wars V, 1980)
Sinister, mature and very well written. The chemistry between Leia (Fisher) and Solo (Ford) is great, as well as the chemistry between Luke (Hamill) and Yoda, who is played by a puppet.
2. Back To The Future (1985)
Science fiction, action, humor, drama, romance and good music. This allround movie has definitely stood the test of time.
3. Aliens (1986)
Action... good action... no, exceptionally good action. And very thrilling.
4. Contact (1997)
Jodie Foster, James Woods et al. in a movie that elegantly aims for the stars.
5. The Truman Show (1998)
A science fiction satire about a very popular reality tv show that is recorded live 24/7 in the largest studio ever built. Jim Carrey is splendid as Truman Burbank, which is a role that is remarkably serious considering Carrey's other roles. The movie is also eerily visionary; compare with modern popular media like reality tv, personal blogs and personal video channels on youtube nowadays.
Drama:
1. Casablanca (1942)
Tight story, great dialogue, entertaining and suspenseful.
2. The Right Stuff (1983)
The story of test pilots trying out new aircraft and trying to break the sound barrier, and the story of the first American astronauts and the Apollo program.
3. Amadeus (1984)
A wonderfully filmed tale about brilliance, envy, romance, plotting and madness.
4. Empire of the Sun (1987)
Great story, good acting, beautiful and engaging.
5. Almost Famous (2000)
Good story, good acting, thoughtful and funny.
Thriller/Action:
1. Raiders of the Lost Ark (1981)
A great, thrilling adventure with a historical and mythological flavor.
2. The Untouchables (1987)
A thrilling police versus gangsters story with great actors.
3. The Silence Of The Lambs (1991)
Thrilling to the bone.
4. Shutter Island (2010)
A thriller mystery story with quite many surprises.
5. The Imitation Game (2014)
A WW2 drama/thriller based on a true story. Great acting.
Other (comedies/animated):
1. The Jungle Book (Disney, 1967)
Great story, great music and entertaining. I love it.
2. The Sting (1973)
A wonderful heist movie with many surprises.
3. Life of Brian (Monty Python, 1979)
A very funny comedy with serious messages underneath it all.
4. Dirty Rotten Scoundrels (1998)
A very funny comedy about two competing con men on the riviera.
5. Shrek (2001)
He's green and the movie is very funny.
Honorable mentions:
Schindler's List, Mission Impossible 3 and/or 4, Nikita, Pulp Fiction, Jurassic Park, Blazing Saddles, Groundhog Day, Lock Stock and Two Smoking Barrels, Snatch, As Good as It Gets, Bitter Moon, Bridge of Spies, Erin Brockovich, Falling Down, Fight Club, Apollo 13, Gandhi, Gladiator, Gran Torino, Little Miss Sunshine, Mississippi Burning, Munich, The Name of the Rose, The Shining, Angel Heart, A New Hope (Star Wars), Alien, Terminator 2 and some other science fiction

.