I don't like the idea of merging all of the branches together. But since that's what the question is I voted Army. The Army has planes, ships, Special Forces, etc etc. They'd probably be the best in absorbing the other branches. I don't think it's a good idea though. But if they were to merge the branches Boot Camp should be to the Marine Corps' standards.
I do however think the Air Force should be given back to the Army.