Ali is one of the most well-known people in the world. Who wouldn't remember a guy who said things like this (in rhyme yet):

I done wrestled with an alligator, I done tussled with a whale; handcuffed lightning, thrown thunder in jail; only last week, I murdered a rock, injured a stone, hospitalized a brick; I'm so mean I make medicine sick." - Muhammad Ali, 1974.
Not to mention his principaled stand against the Vietnam War, which cost him a ton of money, as he lost 4 years of his boxing career and had to appeal to the Supreme Court.

Whoever wrote his wikipedia article says of him:

Muhammad Ali (born Cassius Marcellus Clay, Jr.; January 17, 1942) is an American former professional boxer,[1] philanthropist[2] and social activist.[2] Considered a cultural icon, Ali was both idolized and vilified.[3][4]
and notes:

In 1999, Ali was crowned "Sportsman of the Century" by Sports Illustrated and "Sports Personality of the Century" by the BBC.[11][12]
Ali would go on to become the first and only three-time lineal World Heavyweight Champion.
In retirement, among other things:

On November 17, 2002, Muhammad Ali went to Afghanistan as "U.N. Messenger of Peace".[56] He was in Kabul for a three-day goodwill mission as a special guest of the UN.[57]

On January 8, 2005, Muhammad Ali was presented with the Presidential Citizens Medal by President George W. Bush.

He received the Presidential Medal of Freedom at a White House ceremony on November 9, 2005,[58][59] and the "Otto Hahn Peace Medal in Gold" of the UN Association of Germany (DGVN) in Berlin for his work with the US civil rights movement and the United Nations (December 17, 2005).

As Mrs. Lonnie Ali looks on, President George W. Bush embraces Muhammad Ali after presenting him with the Presidential Medal of Freedom on November 9, 2005, during ceremonies at the White House.
On November 19, 2005 (Ali's 19th wedding anniversary), the $60 million non-profit Muhammad Ali Center opened in downtown Louisville. In addition to displaying his boxing memorabilia, the center focuses on core themes of peace, social responsibility, respect, and personal growth.

According to the Ali Center website, "Since he retired from boxing, Ali has devoted himself to humanitarian endeavors around the globe. He is a devout Muslim, and travels the world over, lending his name and presence to hunger and poverty relief, supporting education efforts of all kinds, promoting adoption and encouraging people to respect and better understand one another. It is estimated that he has helped to provide more than 22 million meals to feed the hungry. Ali travels, on average, more than 200 days per year."
Considering his physical condition, due to Parkinson's, that is even more amazing.