Baron Mordo. This is a better place to start than with Dormammu, telling the story of how Strange and Mordo were both disciples of the Ancient One, and Mordo's treachery and defeat.

Mordo can go running to the Realm of Darkness.... setting up a sequel with Dormammu.

Not sure who I'd cast. Maybe Karl Urban or Clive Owen.

J. K. Simmons as J. Jonah Jameson

 Dormammu - not sure who to cast,probaly just a voice actor and the rest of the performance would be special effects.

I see Hugo Weaving as the main villain of the first movie, Baron Mordo, with Dormammu being set up as the future threat in movies down the line - not sure who would play Dormammu - but it's highly important he not have a British accent.   I don't really see Mordred ever being part of a Dr. Strange movie franchise.

Baron Mordo.  Guy-with-a-firey-face doesn't do much for me in a movie.  (Of course, you could change his look for the screen, but....). 

Slightly odd casting, but I'm thinking Jon Favreau.

Christ, I've just realised that in a toss-up between Mordred and Dormammu I've picked Baron Mordo... Must have read what my brain expected to see.

Of the two actually posed in the toss-up, I'd have to go with Dormammu. But I still think Baron Mordo should be the villain in a Dr Strange movie!