Restricted to Theoretical Physics or Mathematical Physics, you mean?
John Baez is a mathematical physicist - i.e. a mathematician, not a physicist. He says so himself. My vote goes to Steve Carlip, who is the go-to-guy for other physicists in the general relativity discpline. Lubos Motl is also good, in string physics, and Urs Schreiber looks to be the most creative, or at least the one who shares his creative processes best on the web.
Restricted to physics, competancy, and the overall Web.
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