As other pointed out, Chavez looms large in this category. Sweet Pea was denied a rematich, Taylor II occorred way after Taylor's prime, but the most painful one of all, IMO, was Frankie Randall.

Randall defeated Chavez convincingly the first time around and looked like he was on the verge of doing it again in the 2nd fight, ony to have the fight stopped (or Chavez's desire to stop fighting strangely honored) on cuts and somehow given to Chavez.

The third fight took place a billion years later and Randall had already lost a trillion times by then. He didn't have the name recognition for other big fights. the robbery in 2nd fight and effective denial of a rematch really was a cruel thing to do to the man.