From the top of my head, here are some movies I feel is a must see:
Gone With the Wind - GOAT, the casting of the movie was perfect. Vivian Leigh was masterful.
Ben-Hur - 2nd GOAT, to produce a movie like this today would cost more than it can ever take in
Wizard of Oz
Godfather I, II
God, Bad and the Ugly
Conan the Barbarian - I know, but back then having a movie with this much gore and violence was unheard of.
Exorcist - I am still having nightmares about that movie today
Star Wars I-VI
Mad Max I & II
Rocky I-IV Adriaaaannnnnnnnnn
All 5 of Bruce Lee's movie - if you could only see one, check out Return of the Dragon. What can be better than Bruce vs Chuck Norris?
Sword in the Stone
Airplane - "What kind of plane is it? Oh, it's a big pretty white plane with red stripes, curtains in the windows and wheels and it looks like a big Tylenol."
Naked Gun - Hey Look! It's Enrico Pallazzo!
Tombstone - I'll be your Huckleberry
Indiana Jones I-III
If you haven't watched any of these... it might be worth your time to check it out.