@Madmick has a pretty thorough write up. I'd add the following four, as I think stars are made in the heat of battle, and only rise when steel sharpens steel. Without Frazier and Foreman, Ali wouldn't he as revered today. Without Duncan and Nash, neither would Kobe.
Hockey - Wayne Gretzky and Mario Lemieux. Gretzky revolutionized tactics and put that first dint in the glass wall of small men playing pro sports. Before him, hockey was a brutes game for giants, he used skill and smarts to rack up points at such a rate that only one other player has come close. Mario Lemieux had a similar creativity on the ice, and kept a similar pace to Gretzky, until late stage cancer half way through his career forced him out of the game. He luckily managed to win his battle, and even come back famously to win Olympic gold under Gretzky's coaching. But when he returned to the NHL he clearly wasn't the same player and it was obvious that chemotherapy had taken it's toll. Many Canadanian fans today play out the Gretzky/Lemieux stat battle between their proxies McDavid/Crosby, as a testiment to their greatness.
I'm suprised a Disney movie hasn't been made about this tale of two men to be perfectly honest with you.
Rugby - Jonah Lomu and Francois Pienaar. Before Lomu, Rugby was strictly amature, although it was 'amature' in the way the NCAA is. After he retired, it was fully professional. The powers that be decided they would have to go professional just to keep this young man from a poor Pacific islandwr turned star All Black from moving across to Rugby League for a bigger payday.
Juxtaposed to Lomu, was his rival in the 1995 Rugby World Cup. At a time when South Africa had just ended apartheid, voted its first black president, but was on the brink of civil war between hardline white nationalists and hardline black nationalists. Nelson Mandela, trying to rule for the 80% in the middle, recognized the bonding moment sports can bring to a city or a country, tasked the captain of his Springboks with beating Lomu's team and bringing South Africa her first cup. Which he did.
This is dramatized in Clint Eastwood's Invictus, which is a great sports film IMO.
