What Special Forces Group Has The Hardest Training?

Ok so my friend wants to join the military when he is old enough. So him, my other friend, and me we were talking about it and got in a friendly debate about what special forces group has the hardest training. My friend thinks it is the Green Barate (forgive me if i spelt that wrong), my other friend thinks its the Navy Seals, and I am a tie between the Navy Seale and the Rangers. Also could you inform me on other special forces groups and what all of the different groups do? Thank you, your answeres would be apretiated.