Here's some of his most famous fights that you should check out.

Thomas Hearns vs Marvin Hagler
Thomas Hearns vs Sugar Ray Leonard 1 and 2
Thomas Hearns vs Iran Barkley 1 and 2
Thomas Hearns vs Pipino Cuevas
Thomas Hearns vs Roberto Duran
Thomas Hearns vs Dennis Andries
Thomas Hearns vs James Shuler
Thomas Hearns vs Wilfredo Benitez

Mixture of slugfests, great KO's and one sided beatings a good mixture of fights for you to get started on. Also any early Hearns fights you should check out as well. You should be able to find a lot of Hearns fights on Youtube.