I am the Mouth of Sauron.

[Thinlómien]

Personally I think his powers were (far) lesser than Gandalf's or the Witch-Kings. I think his position was more based on his skills of speech and on his intelligence than on his skills in sorcery.

But he was a higher rank than the Witch King . . .

Care to elaborate; what do you base your assumption on? Being the main ambassador or main represantive of Sauron is a very different job from being the Captain of the Army or a sort of steward to Sauron (in Angmar) - they're not very easily compared and they are of pretty equal importance. But if I had to say which one was of "higher rank" I'd undoubtedly go for the Witch-King. (That of course, is my personal opinion. After all, it all comes down to "how do you define 'higher rank'?" ... )