The Graduate - Dustin Hoffman and Katharine Ross after the church scene, they are sitting in the back of a bus wondering just what on earth have they done, to the soundtrack of "Hello Darkness My Old Friend"
The Apartment - Shirley MacLaine realises that it is Jack Lemmon that she is in love with so rushes off to his apartment but then hears a 'gunshot' from inside it.
Silent Running - some people think this is a bit of an odd choice. It's a science fiction film that I first saw as a young teenager as my brother was watching it. I found the ending very emotional, and it's still moving seeing it as an adult. I won't give away the ending as that would be real spoiler.
Oh ... and also a couple of anime films that I would never admit to in public