Good Will Hunting because it dealt with child abuse and how it was not his fault - extremely personal to me and I could not stop crying! I would recommend seeing it for many reasons and obviously it was a tremendous hit.
You know what... I've never seen that movie. Just downloading it now :-)
For me it would have to be Into The Wild. The true story of a young man who left everything behind to live his life alone in Alaska. Very very moving, great soundtrack and just makes us remember that life is what you make it or want it to be.