Gandalf vs. The Witch King? I would have to say Gandalf, since he is a Maia and the Witch King is just a shadow. Sure, he's strong, but he's no match for Mithrandir! PJ made Gandalf look ludicrously weak. I cringed when I saw the way Gandalf fell off his horse and gave The Witch King that helpless look, as if he were saying 'Well. This is the end of me." He was resurrected by Iluvatar himself and he shakes at the sight of a Ringwraith?
And I agree with one of the above comments. In the movie, Eowyn makes Gandalf look like a pansy. Not that she isn't strong, but if she is NOT as strong as a demi-god, then they shouldn't make it seem like that.
I think Gandalf refused to show a good amount of his power. What he demonstrated in the books was like the tip of an iceberg. Maybe even less. After all, he DOES mention in The Two Towers that he is so powerful that practically no one could match him, except 'the one who sits in the Dark Tower'.
This is one of those areas that Tolkien has left very vague. It's another 'Tom Bombadil' scenario where one just doesn't know what to think. One is sure of something, but the rest is just plagued in a cloud, if you take my meaning.
