My favorite movies that most people I meet haven't seen:
The Apartment - 1960, directed by Billy Wilder who is known to you (hopefully) for directing Sunset Blvd and many other things. He's actually my favorite director, and this is my favorite of his films, and in my top five movies, in the number two spot. It rides the line between comedy and drama in a way that I don't generally see (though Wilder is adept at this). The main character is C. C. Baxter, who seeks to get ahead in his gigantic New York office building by allowing his seniors at the firm to use his APARTMENT for their extra-marital affairs. Things get interesting (and then actually quite dark for 1960, but I won't spoil it) when he finds out that his big crush, the elevator girl with a pixie-cut, is sleeping with the head of the firm at his place - and she finds out that said boss is lying about leaving his wife for her. They both get depressed, but eventually sparks fly between them. The main character is a spineless nice-guy-ish character, but in a time period when that wasn't yet ubiquitous, so it makes for an interesting time capsule, on top of being quite funny and heartfelt.
All About Eve - My fifth favorite film. All About Eve is, to me, the movie I've seen with the wittiest dialogue. Verbal fencing has never been so spicy as in this 1950s melodrama of an actress who lies and betrays her way to stardom, weaving a heartless web of deceit. The movie's most sinister (and most enjoyable) character though, is a theater critic, who takes the titular Eve under his wing - and he is voiced by the suave George Sanders, the voice of Shere Khan in the original Disney Jungle Book.
Stalag 17 - Another quintessential Billy Wilder dramadey (before the term existed). The action takes place in a German prison camp in WW2, where a group of American soldiers split their time between killing boredom (by either spying on Russian female prisoners, betting on mice racing, or playing volleyball) and trying to escape the camp. Their escape plans are elaborate, and should be fool-proof, but they keep getting caught - leading to the realization that among them is a German spy posing as an American soldier. Veering on slap stick one minute, and then as tense as any thriller the next, Stalag 17 is a true underrated masterpiece.
Key Largo - Key Largo is a 1940s Humphrey Bogart movie, created hot on the heels of Casablanca. Like Casablanca - Bogart plays a tough guy who sticks his neck out for no one, yadda yadda yadda, but the plot is claustrophobic and, despite containing romance, is not nearly romantic. Bogart plays a solider who has journeyed to the Florida Keys to meet the father and widow of a close war buddy, who own and operate a hotel. When he arrives however, things are amiss - the hotel is empty, aside from a few shady customers who have rented the place out. It turns out that these customers are members of a ruthless gang, with Edward G. Robinson at the helm, as the main villain. If you don't know the name, you certainly know him through cartoon parodies of a gangster saying "Myeah, see, myeah." He dominates the film - and the hotel - as everyone is trapped together for a night while a tropical storm rages outside. Outstanding. To me, it is Casablanca's equal.
All of these movies are currently in my top ten, check em out!