He had a genius for promoting himself. He stands with entertainers like Madonna who probably learned from him.
He had a huge personality, and let it shine through. And underneath his competitiveness and the roughness necessary to his sport, he was a kind man.
Here is a wonderful moment from Allen Funt's Candid Camera program where they asked kids what they would do if they met The Champ. What they didn't know was he was standing behind them.
The best of these begins a minute and 58 seconds in. You can watch the entire video below, or click the link to have it begin at that point. As Funt says you'll see "Five seconds in a life of a kid he'll never forget". I'll bet that man now probably in his 50s is remembering today.