Lord of the Nazgul

[Legolas]

I agree that the witch king is the highest.

There's no need to post agreement with fact. The Witch King being the leader of the Nazgul isn't an opinion.

I am sure the Nazgûl needed a leader, however, why was this one (The Witch King) specifically choosen out apart from the other Eight.

Letter No. 210, speaking of Zimmerman's attempt at putting the work to film:

Their peril is almost entirely due to the unreasoning fear which they inspire (like ghosts). They have no great physical power against the fearless; but what they have, and the fear that they inspire, is enormously increased in darkness. The Witch-king, their leader, is more powerful in all ways than the others; but he must not yet be raised to the stature of Vol. III. There, put in command by Sauron, he is given an added demonic force. But even in the Battle of the Pelennor, the darkness had only just broken.