Depends if you want to be first in and last out, then I would say go in the Marines because they truly are our first ground strike leaders.
If your looking more towards a occupational force then I say the Army, which I was in. The Army are the ones that try to accomplish what the Marines set out to do when in war or on a peace keeping mission
Go in the airforce if you want a fairly comfy life because out of all the services and military installations that I been to the Air Force really has the nicest and some of the more techinical jobs out of all the services
Go into the Navy if you want to see the world and be away from your family for long long periods of time. My mother is a retired Navy Captain who for much of my youth was away on deployments. Also if you like boats Navy is the way to go.
Why anyone would want to deploy to Afghanistan is beyond me, not that there is anything wrong with being patriotic and serving your country, but you really should take to some Iraq and Afghanistan vets who are back stateside now and they will give you the real scoop on what goes down over there.