I terms of significant fights in history it would be

Johnson v Jefferies
Louis v Scheming 2
Rocky v Louis
Ali v Frazier 1
Ali v Foreman
Ali v Frazier

My lifetime

Leonard v Hagler

Barrera v Morales 1
Benn v G Man
Tyson v Berbick
Tyson v Spinks
Tyson v Rudduck 1 & 2
Oscar v Tito
Toney v McCallum
Lewis v Holyfield
Naz v Johnson