Buster vs Tyson is the biggest of all time bar none.
Mentions are definately:
James Toney vs Michael Nunn
Hasim Rahman vs Lennox Lewis
Baldomir vs Judah
Foreman vs Moore (and yes that was hilarious and just what Moore deserved)
Clay vs Liston
Duran vs Davey Moore
Duran vs Iran Barkley (Awesome display by Roberto Duran)
Sugar Shane vs Oscar(one of the first big fights I watched and I picked Sugar Shane)
Ricardo Mayorga vs Vernon Forrest