Fights where the #1 or great fighter lost, typically at the end of their careers, that "passed the torch" to a new generation.
Marciano-Louis
Tyson-Holmes
Holmes-Ali
With Ali you could argue Ali-Moore but Ali Liston was probably more iconic
Leonard-camacho (until ol' chavez came round the corner)
To an extent Tyson-Holmes
Part of me wants to say khan-Barrera but khan didn't really dominate as much as the others
Until danny got beaten by Herrera I would have said danny-morales
Can't think of any others at the moment. Those are off the top of my head.