From your experience, based on supply, demand, and effort to learn -- What skills in today's market as a webmaster have the most value? Second, if you were evaluating another webmaster on that skill, what factors would be the best indicators of their level of expertise?
My take on it: best skill would be "ability to learn fast and evolve", to keep on top of current technologies, and to decide/advice on what is the next big thing and invest in it. (subjective example: should you invest in Silverlight or HTML5 ?).
Next on the list, don't be specialized in a particular platform: Linux/Windows/OSX/mobile are all there.
And if you want to target niches, accessibility expertise is nice to have.