I'd thoroughly recomment Akira Kurosawa's 'Seven Samurai' - it's a fantastic film and seen as the inspiration of many other films, such as the Magnificant Seven and The Guns of Navarone, amongst others.
I first saw this with my dad, many years ago. Our local cinema had a monthly film club, where the more obscure films that wouldn't have got a more commerical release were shown. Watching it always reminds me of my dad.
Do try and get hold of the full 3hr 27min version. There was a digitally remastered full version released in 2006 and it's on my list of DVDs to buy when I see it.
I'd also recommend 'The Killing Fields' - a powerful film.