A.K.A. Cassius Clay (1970) was a documentary made during the time that Muhammed Ali was on "hiatus" from the world of professional boxing due to his strong religious beliefs. The film makers use a lot of valuable achieved footage, interviews with the Champ himself and those that were heavily involved in his life. A lot of classic Malcolm X footage can be found in this film as well. If you want to get a look into the life of the "Greatest Heavyweight Boxer of all time" then I advise you to watch this film. No dramatical theatrics. Just the man himself.
I would also recommend THE GREATEST as well. If you want to get a full picture of Muhammed Ali. I wish that somebody would do a film of his post-Zaire boxing career. Now that would be a fascinating film to see the "Elvis" phase of Muhammed Ali. I've read Ferdie Pacheco's book. Someone in Hollywood needs to make a big screen version of it.
Highly recommended.