Have to chime in with Stew and IndianSummerSky and show some love for Ali. There are so many beautiful moments in that one, but the sequence that always come to mind for me is the scene where Ali goes running through the dusty streets of Zaire, while that slow-burning African music ratchets up the sense of emotion. It's utterly mesmerising, particularly when Ali stops to survey the murals of him fighting off disease, war, etc.
The performances are uniformly excellent, and the scenes showing the relationship between Ali and Howard Cossell (all banter and verbal sparring on camera, true friends in private) are just great. I love the beautiful, understated scene where Cossell phones Ali to tell him the grand jury has unanimously decided not to send him to jail.
As for why the film doesn't receive the admiration it deserves? I think one contributing factor is that When We Were Kings covered the same subject matter so brilliantly. Consequently, you could say that in a strange way Ali's attention to detail works against it, and that at times individual scenes, and by extension the movie as a whole, feels like an incredibly adept recreation of something already definitively captured by a documentary. Or rather, that's how it might feel to a casual viewer. I look upon the two movies as excellent companion pieces.
Also, if memory serves, the theatrical tagline for Ali was 'forget what you think you know' (or something very similar), which rather misleadingly suggested a more controversial movie examining the cultural icon in a new light.