Each branch of the militaries ranks basically follow the E1 - E9 rankongs for enlisted personel (along with Specialist 4 and 5 in the Army), Warrent Officers 1 - 4, and then Officers which I can't remember how they are reffered to. Each branchs ranks, although they may be called different names depending on the branch, follow the same structure. So an E7 in the Army would outrank an E5 in the Navy, Marines, or Air Force. Same thing with officers however you would have to check a charting of each branches officers to see who outranks who. (I was an elisted man, never made it past E5) zslane and Spence are right on much of what they have said, although I'm going to assume your situation is some time after the beginning of the apocolypse started, laws would no longer apply to the military not operating in the country. The highest ranking officer would be in charge regardless of the branch he was in. Spence is correct that most
likely it would be National Gaurd personnel, but there are plenty of Army, Marine, Navy, and Air Force personnel all over the US, so having any number of them from any branch is feasible. However, a Colonel may be a more plausable rank for the ranking officer. There are more of them around than there are Generals. Just a thought! Hope some of this may help with your ideas! Have fun!