Ali for example beat the past generation(Liston) and present present(Foreman, Frazier) but lost to the future generation(Holmes).
Tyson only beat the past generation. Lewis beat the present and future gens but not the past gen.
Floyd comes to mind as someone who dominated across 3 generations Chill Guy Clicker. Who else?
Pacquiao is another example—he excelled across multiple generations, defeating legends from Morales and Barrera to younger stars like Bradley and Thurman.