If you call the 'Voice of
Saruman' a display of power, then I guess we could quite easily say that
Gandalf would win against the
Witch-King of Angmar.
Unfinished Tales
"Two days after Gandalf had departed from Orthanc, then Lord of Morgul halted before the Gate of Isengard. Then Saruman, already filled with wrath and fear by the escape of Gandalf, perceived the peril of standing between enemies, a known traitor to both. His dread was great, for his hope of deceiving Sauron, or at the least of receiving his favour in victory, was utterly lost. Now either he himself must gain the Ring or come to ruin and torment. But he was wary and cunning still, and he had ordered Isengard against just such an evil chance. The Circle of Isengard was too strong for even the Lord of Morgul and his company to assail without great force of war. Therefore to his challenge and demands he received only the answer of the voice of Saruman, that spoke by some art as though it came from the Gate itself.
'It is not a land that you look for,' it said. 'I know what you seek, though you do not name it. I have it not as surely its servants perceive without telling; for if I had it, then you would bow before me and call me Lord. And if I knew where this thing was hid, I should not be here, but long gone before you to take it. There is one only whom I guess to have this knowledge, Mithrandir, enemy of Sauron. And since it is but two days since he departed from Isengard, seek him nearby.'
Such still was the power of the voice of Saruman that even the Lord of the Nazgûl did not question what it said, whether it was false or short of the full truth; but straightaway he rode from the Gate and began to hunt for Gandalf in Rohan."
Great indeed must the power of the voice of Saruman be, if he can convince the Lord of the Nazgûl to take his word for it, when there was a definite chance that he was lying to steer them astray. And don't forget the errand of the
Úlairi, to retrieve the Ring! If Saruman got it then Sauron would indeed be vanquished, so, to trust Saruman on such a matter of supreme importance would require great power on the part of Saruman. When Gandalf becomes White, he is greater than Saruman, and therefore by logical deduction, Gandalf too, must be more powerful than the Lord of Morgul. However, to me, this seems like a weak argument.
A far better one is the fact that the Lord of Morgul was enslaved into Sauron's service through a
Ring of Power. If Gandalf or Saruman possessed one of
the Nine Rings, then I believe that they wouldn't be overcome by them. I mean, c'mon, they perceived that they could use
the One Ring, and therefore must be more powerful if they can wield it, and not become subject to it. It was conceived that
Glorfindel could take the
Witch-King on. Gandalf makes a reference to mortals and Great Rings in
The Shadow of the Past, he never mentions immortals. He therefore makes a distinction between the two. The Witch-King was a mortal, and was therefore of lesser stature to Gandalf (and the
Elves). But, the power given to the Witch-King by Sauron would indeed be another matter. But this quote says it all.
The Letters of JRR Tolkien: Letter #210 from a letter to Forrest J. Ackerman, June 1958
"It is based on the misconception of the Black Riders throughout, which I beg Z to reconsider. 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. See III 114."
I would say that most of the people of ME would indeed fear the Nazgûl (they'd scare the sh*t out of me!
), but I wouldn't think that Gandalf feared them, not even the Witch-King, and therefore as they have no great physical power against the fearless, then, undoubtedly, he'd win!