Ali's greatness? Hard to be objective. At his best/near best, a superb athlete, with marvellous movement and timing. Ditto to everything Kendall says, although of those I have seen, a few stick in the mind:
Sugar Ray Leonard - surely second to none for timing, movement and skill. Not fair to compare him to Ali though, because he fought at a lower weight.
Of the heavyweights, it is easy to forget what a marvellous champion Larry Holmes was in his prime. He never ducked a legitimate challenge and he held the title for over nine years. No one else has done that in my lifetime. He ultimately came unstuck on the same flaw as Ali. He was as good as he thought he was, but not for as long as he thought he was.
Lennox Lewis defeated every man, who ever entered the ring with him. Both of his defeats were down to being under prepared. He absolutely flattened both McCall and Rahman in the rematches. He had the boxing skills, athleticism and he grew in power as a puncher as his career progressed.
"The Greatest"? It's impossible to judge. Still, he (Ali) is certainly the greatest to many people and I can see why.