-Hardest players to defend based on their skills:
Melo,Kobe and then comes Durant
Durant's length helps him too. As for Melo, he is stronger than durant, but durant makes it up with his enthusiasm/hunger.. When durant builds some more muscle, he might easily surpass lebron, that would be scary as fukc!
As for kobe, he has been a hardworking player all of his career, we dont see many skillful players who work on improving themselves everyday like he does. He doesn't have the length of durant nor the body strength of melo, but he makes it up with his will.
-Hardest players to defend based on their bodies:
LeBron and Dirk
Obviously lebron and dirk have skills, but lebron's athleticism helps him a lot. Dirk's length helps him hugely, plus he is skillful enough to shoot the ball without any care about defense due to his length.