I've been thinking about this, and for some reason no dead actors are coming to mind. Except that I don't know whether Harry Jones is alive or dead - he's not had a credit on IMDB since 2000 - and I think he'd have been a very good Doctor in a Troughton/McCoy mould: looks like an idiot but more powerful and dangerous than anyone thinks. So I was amused to find his face in this thread already, even if he was there supporting Servalan!

Of the living... not sure why that immediately puts me on to women... altariel once made the very good suggestion of Fiona Shaw (as in "a great actress who does a lot of crap so wouldn't say no on the grounds it's a kids' programme"). And I thought Melissa Jaffer's Noranti was very Doctorish, in that schoolmarm-turned-baglady-on-the-hippy-trail sort of way. Oh, and if I'm thinking of dead women, Mary Morris, who was such a stunning Number Two in The Prisoner.