I've always thought of myself as someone who's seen a lot of films, but after a recent talk with a film historian I am absolutely embarrassed. What are some great and essential films that aren't quite in the 'mainstream' that I need to watch? Foreign as well?

What are some of the absolute best movies that must be watched by anyone interested in film?

I've heard the names Bergman, Tarr, Kurosawa, and Tarkovsky thrown around, so I suppose I'm looking for films in their direction.

What are the essential foreign films? English films? Classics? Silent films? etc. That I should watch as soon as possible?