Army... by far. The Army has 2450 casualties. The Marine Corps follows with 872. The reason of so many army casualities is because the army is the biggest military branch therefore more deployment and more people to kill.
More information can be found at http://icasualties.org/oif/USDeathsByService.aspx
U.S. Army
The US Army has suffered the most casualties in the Iraq and Afghanistan.
According to the Department of Defense there have been approximately 4,500 Casualties on account of Operation Iraqi Freedom.
As of December 15, 151 coalition personnel have been killed in Iraq and 494 in Afghanistan. Civilian casualties are subject to estimates that vary widely. Enemy casualties are generally not tallied or reported in detail.
People and soldiers engaged in war.
You'll have to monitor those statistics; as those are probably still being collected for US Women/Men military casualties.
According to the Senate website, in the current congress, there are 26 senators who served in the military. These include senators who served in Korea, Vietnam, Kosovo, Iraq, and Afghanistan.
The United States Army has had the most deaths while in Iraq with the Marine Corps following up behind it, however that is only because the Army is the biggest branch of the military. Each branch have experienced losses, including the Coast Guard, but the Army has received the most.
ALL of US military branches are currently seving some part in Iraq.
Iraq under U.S. Military Occupation happened in 2003.
the war in Iraq ended today. August 31st, 2010. there were 4,400 U.S. casualties
During World War 2 the total casualty lost by Iraq were only about 500.
http://www.defenselink.mil/news/casualty.pdf