My educated guess...
big forearms for gripping boxing gloves easily
legs for improved mobility and power transfer
chest for straight punches being strong in the clinches
biceps/shoulders/back/latts for looping punches like Marcos Maidana's overhand right and uppercuts
abdominals for head movement
What are the most important muscles for a boxer?
http://i.imgur.com/sHi117N.jpg
Given that the peck deck/seated fly is an important exercise for pectoral muscle, I'd say they're just as important for hooks as they are for straight punches.
I know that cardio is a necessity, hence why boxers don't overdevelop their muscles. They use a lot of cardio. The first time I hit a heavy bag I was surprised at how much cardio it took. I was gassed out quickly!
http://pinoysuperstar.com/wp-content/images//336x252xpre-fight-01-2006-11-18.jpg.pagespeed.ic.6xNt14JVA4.jpg