I would like to amend my previous statement and say that all this debate over the best is ridiculous in the extreme. Myself being in the Army, naturally I am going to have pride for that branch, however from a third party objective perspective I believe that every branch is elite in its own right. The US Military encompasses all of these Special Operation Teams for a purpose. Each team is designed for a specific need. Seals are unsurpassed at underwater and demolition, specifically all SEAL teams are well versed in nuclear arsenals and have the capability to deactivate them. Rangers are great at Direct Action/Urban Assualt. US Army Special Forces ( Green Berets ) are great at Foreign Internal Defense and Counter Terrorism. Marine Force Recon are great at reconnaissance behind enemy lines, survival without large scale military support, and have great shooting/sniper teams. Delta is the go to counter terrorism team of the US Military and is great at taking down airplanes, and rescuing hostages.
Spark Notes: There is no best, just apples and oranges. Whatever the situation is, the US Military has a unit to respond to it.