The Dollars Trilogy. (A Fistful of Dollars, For a Few Dollars More, The Good, the Bad, and the Ugly) They are all separate movies - no continuing story and some characters who have died previously come back in another movie. Depending on how you like Lee Van Cleef as a good guy or bad guy, you should watch the second movie before the third if you like him good or the third movie first if you like him bad.
Personally, I recommend you to watch the second movie first. My favorite of the three masterpieces.
Other movies you could watch: Hot Fuzz, The first two Godfather movies, Young Frankenstein, Batman: Under the Red Hood, the two original Batman movies with Beetlejuice as the main character, Gone with the Wind (the outlier of the list), Apocalypse Now, Dracula (the one with Bela Lugosi), Goodfellas, Dr. Strangelove, and Magnum Force.
You should also check out this wikipedia page for any other ones.
http://en.wikipedia.org/wiki/National_Film_Registry