Stark wrote:Does anyone have any idea where the Witch-King's superiority comes from? Was he the greatest king? Is his ring better? Is it because he's a sorcerer/necromancer/wizard/etc?
I believe that, in life, he was the mightiest of the lords corrupted by Sauron. From what I have gathered (and, admittedly, a great deal of speculation) he was a great Numenorian of royal lineage that was ruling over holdings in Middle-Earth.
Stark wrote:Do vanilla Nazgul have magic powers beyond their innate Ringwraith stuff? I'm curious, because regular Nazgul aren't a problem for Gandalf (even several), but the Witch-King alone was a serious threat to the guy that killed a Balrog.
The powers of the Nazgul are based on three core principles: Innate power, power gained from the ring, and the power of their master.
The first one is the reason that Sauron gifted the rings to "powerful kings of men." After all, why settle for weak servants when you could have strong ones? In addition, it is stated quite clearly that the powers of the Nazgul vary from Nazgul to Nazgul, with Khamul being the second most powerful of the Nine. (though, yes, he was severely weakened by the light)
Secondly, the ring powers. These are much more vague, but it seems to be a power enhancer/multiplier rather than just a few set abilites you gain. The books speak of the kings becoming greater in all their works prior to their corruption, in addition to gaining great wealth and virtual immortality, so they seem to support this hypothesis.
Thirdly, the power of Sauron has some bearing on the power of the Nazgul. How much? Very difficult to say. I put forward that they remain just as personally powerful regardless of the waxing/waning of Sauron, but merely have no support base or places to fall back to should they be discovered by the Wise, and hence must keep a much lower profile. This theory is supported by the Witch-King's emergence and rule over Carn-Dum (Third Age ~1300-~1975) and war with Arnor despite Sauron's defeat in the Second Age and failure to re-emerge until year 1980 of the Third Age.
Stark wrote:I figure it's due to his status as a proper magic-using wizard
Perhaps, in a sense, but that distorts the concept a bit. Beings in Tolkien's universe seem to have more of an innate "Power" than set spells and wizardry. When using "magic" it seems to be more an extension of that power than, say, the invocation of astral powers via ritual that we see in D&D. I hypothesise that it was the Witch-King's actual level of power (innate plus what he gained from the ring, of course) that allowed him to stand against Gandalf, rather than merely the fact that he had "spells" to fight him with and the other Nazgul did not. A third factor that might or might not have been present was the will of Sauron being focused on the area, and perhaps dampening Gandalf's powers. I have, however, no way to support that, and it is little more than speculation.
Stark wrote:or because Sauron provides more of his power to the Witch-King through his ring, but I've never seen a source for either possibility.
I have never heard of anything like that. In fact, it runs counter to several sources. I have no doubt Sauron is capable of empowering his forces, (hell, it is demonstrated that Melkor could) but I think it more likely he does it through bending his will to that task rather than using the rings as a channel.