i don't think the phrases "best film" and "not mainstream" go together. like i said, impossible task.
(edit) either way, i don't think the big lebowski is that mainstream... i know tons of people who have never heard of it.
omg yes. cidade de deus is my favorite foreign film, and one of my favorite films of all time.
here are some to consider:
magnolia (my favorite movie)
city of god (already mentioned)
amelie
boogie nights
pulp fiction
the big lebowski (greatest comedy ever)
other people have mentioned...
are you sure? i thought protestants, catholics, and lutherans were all demoninations of the Christian faith. i've never heard of anyone who was "just a Christian". sure, all these religions have varying differences but they all follow the same basic Christian belief.