On 21st July 1927, former heavyweight champion Jack Dempsey beat Jack Sharkey by way of technical knockout. Sharkey would win the title five years later in a controversial decision against Germany's Max Schmeling. Dempsey would fail in his attempt to regain the title against Gene Tunney and retire. This is an example of a former heavyweight champion beating a future heavyweight champion. But what is the greatest such win? My vote would be for Max Schmeling beating Joe Louis. Note: By former HW champion, I mean a heavyweight champion who had lost his title and would never win it back. By future HW champion, I mean a heavyweight champion who had not won any HW titles going into that fight. So, Frazier beating Ali, Ali beating Foreman or Foreman beating Frazier all don't count. Lewis beating Vitali does not count, neither does Ali beating Norton in 1973 (Ali would win the title again in 1974). Louis or Charles beating Walcott also don't count as they were holding the title during their wins.
Oliver McCall lost his heavyweight title that he had won against Lennox Lewis to Frank Bruno. In his next fight, he knocked out undefeated Oleg Maskaev who then went on to knockout Hasim Rahman almost a decade later to win the WBC belt.