Actually, without a Life Orb/Choice Specs, Nidoking's bad special attack stat shows, and without a Choice Scarf, his Speed is also bad. On top of that, there's not a lot of GOOD TMs that Nidoking can learn right now. I know Sludge Wave and Earth Power are nothing to laugh at, but Nidoking needs more coverage.
That doesn't mean Nidoking's bad, it means that he's not so good right now.
I'd say: "C" right now, and when items/TMs become available, "A"