Adolf Hitler: The Greatest Story Never Told, a marathon 6-hour-long documentary by Dennis Wise providing a full biographical account of Hitler's life.
There are many documentary films that have examined Hitler's role as ruler of the Third Reich such as Hitler in Colour. When it comes to the life of Hitler starting from when he was born, few even come close to "The Greatest Story Never Told". This film was made in 2013 and features an enormous amount of archive footage to accompany the extensive research by Dennis Wise.
If you are looking for a documentary that gives an unprecedented look into a man that changed the world forever from birth to death then this documentary is a good place to start.