David Remnick in The New Yorker:
What a loss to suffer, even if for years you knew it was coming. Muhammad Ali, who died Friday, in Phoenix, at the age of seventy-four, was the most fantastical American figure of his era, a self-invented character of such physical wit, political defiance, global fame, and sheer originality that no novelist you might name would dare conceive him. Born Cassius Clay in Jim Crow-era Louisville, Kentucky, he was a skinny, quick-witted kid, the son of a sign painter and a house cleaner, who learned to box at the age of twelve to avenge the indignity of a stolen bicycle, a sixty-dollar red Schwinn that he could not bear to lose. Eventually, Ali became arguably the most famous person on the planet, known as a supreme athlete, an uncanny blend of power, improvisation, and velocity; a master of rhyming prediction and derision; an exemplar and symbol of racial pride; a fighter, a draft resister, an acolyte, a preacher, a separatist, an integrationist, a comedian, an actor, a dancer, a butterfly, a bee, a figure of immense courage.
More here. And here's what I said about him on 3QD eight years ago.