I think the most famous of all was Sonny Liston, invincible monster one year to embarrassing chump the next. He continues to fight for five more years yet he is not a contender to fight Ellis or Frazier for Ali's vacant crown? I know Liston had his problems but his invincibility vanished after the Ali fights.
I think Foreman to a lessor extent in the 70's, he was only 28 when he retired the first time around, very young for a heavyweight to finish.