Delta Force
Counter terrorism, hostage rescue, intelligence, high profile recon and direct action.
DEVGRU (Navy Seal 6) = same same as Delta in terms of job. Delta is Army and NS6 is Navy. They perform similar functions.
Green Berets
Force Multiplier - Go into a warzone, educate and train the locals into a fighting force. They are also direct action and guerrilla warfare experts.
Navy Seals and SAS
Versatile small unit opts and/or covert opts.
Army Rangers
Versatile Light infantry especially used for raids, seizing airstrips etc.
USMC
Infantry (tougher training than standard army infantry)
Army
Infantry
Who takes the cake?
As you can see they all specialise in different areas, but I'm inclined to give Delta Force the thumbs up.