I was thinking this, too. Walkout music is a relatively recent gimmick, though it could be argued that it's a 40+ year old tradition. Still, relatively recent in the grand history of the fight game.
Not sure what mine would be but I always loved how Hopkins would come out to It's a man's world by James Brown. The song seemed to have special meaning to an ex con that came up the hard way in this sport. The guy took a lot of pride in his true toughness.
I would have gone for Clannad's ' Theme From Harry's Game '. I always loved Leicester Flyweight walking out to Otis Reeding's ' Try a Little Tenderness '.