It really depends how hardcore a fan your dad is. If he's like most dads, a casual fan who only really knows the big name fighters then two dvd's stand out a mile.

Champions Forever featuring Ali, Foreman, Frazier and Holmes imo the greatest boxing doc of all time.

When We Were Kings, documentary on Ali leading up to the Rumble In The jungle.

The only reason not to choose these two would be if your dad already has them as they are the best boxing doc's ever made imo