This could cause a bit of debate this topic on the exact meaning of "past prime".. But as an example this is what Im talking about...Tommy Hearns vs Dennis Andries. A few years after the Hagler war, miles away from his best weight but Tommy put on a beautiful and dominant performance against a pretty handy titlist, who continued to prove his class in the next few years. One of my favourites along with Harada-Famechon I.
That ranks pretty high on my list aswell...Gets a tad overrated because of the overhyping of Pavlik but the level of domination makes that one special.
George Foreman vs Michael Moorer. Thomas Hearns vs Virgil Hill Jimmy Paul vs Todd Foster Dennis Andres vs Jeff Harding II Corrie Sanders vs Wladimir Klitschko
Holmes Vs. Mercer (who was undefeated at the time) Duran Vs. Moore Duran Vs. Barkley Foreman Vs. Moorer (That single punch was classic!) just a couple...
Leonard - Hagler Frankie Duarte - Albert Davila (Don't know if Frankie ever had a prime because of other issues) Duran - Moore, Cuevas, Barkley