So Gandalf was one of the immortals, an istaris and Ring Bearer but the Witch King was a sorcerer and a Wriath, Ring Bearer and backed by Sauron…

So who was the more mighty?

Certainly, there is a discrepancy in their encounter between the book and the movie adaptation.

In the movie Gandalf seems to get a bit of an ass whipping while in the book, while he is certainly not unaffected, he is able to stand his own against the best the Witch King had to offer.