What made me think of this was bhop after his win against tarver. He should retire now on a high. H faces a big chance of a loss against winky and a further bruise to his legacy.
Others who should have quit on a high but didnt include,

Morales after first pac fight.
Ali after third frazier fight.
Trinidad after Mayorga.
Foreman after Moorer

There are heaps of others. Any other obvious ones?