Sauron's knowledge of the Istari

[Lord of Angmar]

he meeting of Gandalf and the Witch king at the gates of Minas Tirith seemed like a contest of wills between two equals yet Gandalf was most likely the much more powerful one.

Gandalf was indeed immensely more powerful than the Witch King, but, even in the most dire need of Middle Earth, he could not reveal his true power to the Witch King and the mortals around. Also, the Witch King was guided and controlled by Sauron, who I am sure lent a great deal of power to his Captain.