Reposting my previous suggestion for people who don't want to read the entire thing, Gandalf the White vs the Witch King.
Witch King obviously
Honestly it depends on many factors; Gandalf is limited in power while he's in Middle Earth, WK is not (not limited with how much could he use whatever power he has).
WK's power is sort of directly derived from Sauron's own strength and will, so more Sauron = more WK. WitchKing has also a damn ring of power
WitchKing can use black magic and such to harm and kill (that's where the "witch" came from).
Now forget about the magic and all. if it came to pure melee 1 on 1, I would give the edge to the WitchKing.
Gandalf is powerful, true, but WitchKing is a more experienced fighter. Just think of how many battles he's been in since before he was even a Nazgul (since the days of Numenor (?)). Besides, he's got a huge arsenal of weapons to force his opponent to fight of his terms; An awesome sword to kill, a bigass mace to crash shields and armor, poisonous blades and knives...
Oh, and just to counter whatever argument you may come up with, the Witch King can only be killed by girls (as humiliating as it sounds), so a Gandalf vs WitchKing? the odds are Witch King will always survive to tell about it