I may have already mentioned this, but I think that guy who plays Duke on Haven might be a good candidate.
Just because he's skinny and has a mustache? Nah, I don't get a Sorcerer Supreme vibe from him at all. He usually plays wise-cracking, blue-collar types . . . which is not Stephen Strange at all.
Maybe Zachary Quinto or Benedict Cumberbatch? (Or however you spell it.) You need somebody who can do cultured, aloof, and otherworldly. Probably a Brit.
Daniel Radcliffe? (Unless he's tired of playing wizards.)
Somebody at io9 just suggested David Tennant, which is also not a bad idea. You need somebody slightly off-kilter and mysterious . . . .