[Hot, crispy nice hobbit]

Did the Nazguls really ceased to be Man? Its said that they faded. Seems pretty dubious whether they really "died" and became ghosts. On the physical nature of spirits, Tolkien seems pretty clear. Houseless spirits could not wear Rings. Sauron in spirit form could not rescue his Ring after being cut off by Isildur. He must transform into a more tangible form before physically being able to interact with objects. Even the spirits of dead Elves only reincarnated as Elves, with the exception of Luthien (But then, that was Eru's call). The Nazguls were never Maia; so they could not transform into, say, Huge Red Eyeballs.

The spirits of Edain were said to be beyond the Fate weaved by the Music of Ainur anyway... a gift of the Illuvator. Unless there's prove otherwise that Sauron's power extend beyond death (Quite a feat! Hurin taunted Morgoth that he could not enslave Man's spirit beyond the circles of the world), I'd say that the powers of a Nazgul's its own, albeit increased somewhat by Sauron.

Oh yes, back to the question. WK was undoubtedly the chief instrument of Sauron, probably something of the Right/Left index finger. I'd suppose WK's second in power only to Sauron among his slaves. Yep, even the mighty Oliphaunts probably give him a wide berth... But among the Free People... Hard to determine, since he got killed in direct combat by some two-penny worth of a girl and a sprinkling of midget with hairy feet...

"Keep out: Black Breath" - WK's lodge