In my opinion the guy with the best resume in boxing, at least in terms of who he's fought, if not always winning is Evander Holyfield.

Look at his record.

Tyson twice
Lewis twice
Bowe 3 times
Foreman
Holmes
Moorer twice
Qawi twice

Even past his best in mostly losing efforts he still managed to fight the best that was out there

Byrd
Rahman
Toney
Ruiz
Ibragimov

Plus good wins over Buster Douglas, Michael Dokes etc.

No heavyweight in the history of boxing save for possibly Ali has fought as many great fighters as Holyfield.