The Good, The Bad, and The Ugly, starring Clint Eastwood and some other guys. I'd recommend it, as it's probably my favorite movie.
You should start with
A Fistful of Dollars. But why would you watch that when you could watch
Yojimbo instead? They're both essentially the same movie, but Kurosawa is a superior director.
Speaking of directors, I'll give you a list of a few directors whose films I would recommend. I'll also include a suggestion of which of the director's films you should watch with each name.
Bergman -
The Seventh SealScott -
Blade RunnerLang -
MScorsese -
Taxi DriverBeatty -
RedsWelles -
Touch of EvilKubrick -
2001: A Space OdysseyTarkovsky -
SolarisCoen -
FargoI can recommend more films, especially if you're more specific about what sort of films interest you, but this is a nice mix that will expose you to a wide range of genres and cinematographic styles.