The Marine Core.
They're the first to go into the battle, the one's who prepare the way for the rest of the military, they have training longer and harder than any other branch of the military. They make every bullet count and with a U.S. Marine, firing one shot to take out the enemy is preferred over firing a rainfall of led.
The honor of being a Marine is great as well. How many people can say they're a Marine? In the Marine Core, there is no individuality, everyone is a brother/sister and if you take two Marines from completely two different places of training and completely different elements of fighting, they'll be able to work together.