I think overall the Marines are the best. (I was USAF)..
The reason? The Marines have pretty much all they need within their own service for combat. They have their own dedicated assault ships, landing craft, copters, attack planes, tanks, artillery, troops, etc. All the other branches only have part of the whole and have to rely on another branch to provide some combat capability. For example, the Army needs the USAF for air support. The Navy needs the USAF for long distance air cover and support, The USAF needs the Army for base defense, the Army needs the Navy and USAF for transport of heavy cargo such as the M1 tank. Only the Marines have all that it takes to go fight. (Yes, the Navy does supply medical support but that is not really a combat arm and I am talking about what you really need to go fight.)
Simper Fi, be the best Marine you can.
(COMMENT: The Coast Guard is part of the Department of Homeland Security, not the DOD. While it has military capabilities, it in general is a defensive service and not one that usually gets involved in offensive military actions. I did not consider that in your question as I do not consider the Coast Guard as part of the military.)