I would rate the American army rather low for efficency. Consider the last 3 major engagements they've been in- Vietnam, Gilf War, and Iraq. Without going near Vietnam..
1. Gulf War. A 6 month build up followed by a 6 week bombing campaign. What other campaign in world history has given the attacker over 7 months to prepare the battloefield?
2. Iraq. Beating aup a poorly trained adnd equiped army crippled by 12 years of sanctions.
I rate the American Airforce and Navy however entirely differently.
I would rate the German army of WW2 as the best modern army. They lost but it took the USSR, America and Britain and the rest 6 years to do it. Population wise the Germans were outnumbered by at least 5 to one and England alone made more tanks than Germany. when you add in the fact Germany had a minimal Navy and basicly built its airforce in 6 years (33-39) you start to get an idea of how well the average German soldier fought. And they still just about won (and probably would have had Hitler not interfered in military decisions).
Looking at American military history I would rate the Americans as a country as average in terms of fighting ability. They constantly underesitmate their foes (Japanese, Battle of the Bulge, Vietnamese, Iraq, and probably the Soviets as well) and usually win their battles through firepower and technology (post 1945 or so).
The modern American MILITARY (not army or marines they all support each other) is the best in the world. Definatly not the most efficent though.