What is the hardest military branch?

What military branch do you consider to be the most challenging, both physically and mentally? Each branch of the military has its own unique demands and requirements, so it's interesting to hear people's perspectives on which one they believe poses the greatest difficulties. Do you think it's the Army, with its extensive training and deployment requirements? Or could it be the Navy, with its long periods away from home and the demanding nature of naval operations? Alternatively, perhaps you believe the Air Force or Marine Corps poses the greatest challenges, given their unique missions and operational environments. What factors do you consider when answering this question?