Who was the first challenger to defeat Muhammad Ali in a World Heavyweight championship contest?

All four fighters defeated Ali but only Leon Spinks was challenging him for the World Heavyweight Championship. Ali won the world title in 1963 defeating Sonny Liston. He successfully defended the Heavyweight Championship nine times before he was stripped of the title, for refusing to be inducted into the US Army. During Ali's absence from the ring, Joe Frazier became Heavyweight Champion defeating Buster Mathis in 1968. Frazier was the first professional boxer to beat Ali when they met at Madison Square Garden in 1971. However, Frazier was officially Heavyweight champion and Ali was the challenger. Ken Norton defeated Ali in 1973 but was challenging him for the North American Boxing Federation title and not the World Heavyweight Championship. In 1974 Ali regained the Heavyweight Championship when he defeated George Foreman in the famous "Rumble In The Jungle." In 1978 Ali defended his World title against, former Olympic Gold Medallist, Leon Spinks. The young challenger became Heavyweight Champion when he defeated Ali but lost the subsequent return match. Following a short retirement, Ali unsuccessfully challenged, the then Heavyweight Champion, Larry Holmes in 1980.