[Jack Hammer]

According to various sources, the 442nd Regimental Combat Team (Nisei) became the most decorated unit in U.S. military history for its size and length of service, earning it the title "The Purple Heart Battalion." Not sure how that compares to the Marine Corps - for example on Peleliu and Iwo Jima - but it's certainly nothing to sneeze at.

[Jedi Master Pikachu]

Granted, this is from Wikipedia: http://en.wikipedia.org/wiki/442nd_Regimental_Combat_Team , but it has the Nisei Regiment as the most decorated for how long it lasted and for the amount of people it had. So, the Marines could be the most decorated over all.