Why would you think that? Tszyu didn't have 13 fights in his career. He was 13-0 going into the Rodriguez fight.
Leon Spinks won the heavyweight title in his 8th fight. Sven Ottke one a super middleweight belt kn his 13th.
those are probably the winners then. Unless we’re talking “ linear” and not just title holders in which case it probably goes to Leon Spinks .
Jeff Fenech won his first championship belt in his 7th fight (it was the then pretty lightly regarded IBF Bantamweight title).