Ali has several memorable post prime perfomances, so I stayed clear of him altogether (also because I post so much about him otherwise, anyway). I suppose the win over Foreman is his most impressive post prime perfomance, though. But I do probably rate Leonard's win over Hagler as greater. Hagler was unlike Foreman not in his physical prime, but naturallly bigger than Leonard and a much more complete fighter than Foreman.
Pernell Whitaker v Oscar De La Hoya Roberto Duran v Moore/Hagler/Barkley James Toney v Vassily Jirov Thomas Hearns v Virgil Hill Mike McCallum v Toney/Harding Azumah Nelson v Gabe Ruelas & Jesse James Leija
Toney-Jirov is a good one. Also, Leonard-Lalonde. Those combos Ray finished Lalonde off with were vintage Leonard.
Not quite the last... See Nelson-Ruelas II and Nelson-Leija III. All three are great post-prime performances.
Dick Tiger vs. Nino Benvenuti is one of my personal faves. Easily the Hopkins-Pavlik of its era. Also... Jr. Jones vs. Barrera Vince Phillips vs. Tszyu Frankie Randall vs. JC Chavez I, Coggi I Glen Johnson vs. Roy Jones Has anyone mentioned Holyfield's upset of Tyson?
Wilfredo Gomez vs Rocky Lockridge (although Gomez was fortunate to get the nod still a quality performance by a man years past his peak and two weight classes higher.)