My favorites are (in no particular order):
ET (1982) A great emotional story, very well told

The Wizard of Oz (1939) The Technicolor is amazing, and the effects are still amazing in a digital world.

Casablanca (1941) This is one of the greatest classics. There are so many lines that are famous from this classic, I lost count.

Close Encounters of the Third Kind (1977) A serious look at the UFO phenomena through normal people.

The Empire Strikes Back (1980) The best of the Star Wars films, and one of the best ever made.

Pulp Fiction (1994) A very new look at an old filming style. Rich characters and great dialogue. Throw in Tarantino's Kill Bill Part 1 and 2, which was very entertaining.

Fargo (1996) Joel and Ethan Coel are among the best film makers out there. This movie is their best (though Blood Simple gives it a run for the money).

Blazing Saddles, Young Frankenstein, and Silent Movie (1974 to 1976) Mel Brooks made three great comedies in a row. It is hard to choose any one of these as his best. All three are a must see.

Airplane (1980) The Naked Gun (1988) These movies are filled with goofy humor and Leslie Nielsen makes the latter series.

Monty Python and the Holy Grail (1975) While on comedy, this is one of the best out there. NIII! NIII!

This is Spinal Tap (1984) Waiting for Guffman (1996) and the others by Christopher Guest and friends. These movies are mostly improvised and quite hilarious. Guffman is my favorite of the movies.