I think it's Admiral Hargreaves. Robert Brown did a good job as M, but his characterisation wasn't exactly the same as Bernard Lee. At the time I didn't think about whether this M was the same person as the previous one, because we'd got used to one actor followed by another playing James Bond - and numerous actors playing the likes of Leiter or Blofeld - that it seemed quite possible that the same "M" could be played be more than one actor.
But the casting of Judi Dench changed that, for obvious reasons. (Indeed it would appear that Dame Judi, by appearing in the Brosnan and Craig Bond films, was playing two different people who happened to be "M" - the names Dame Barbara Mawdsley and Olivia Mansfield have been bandied about.) And now for Bond 24 we very definitely have a different man in charge as M, former SAS Colonel Gareth Mallory - and I think that having an ex-special forces officer as Bond's new superior is quite a smart move.
So now, I think of Robert Brown's M as Admiral Hargreaves, late of the RN, where I think he was in charge of the submarine force, then head of MI6.
One point which the Craig films corrected regarding M's superior. For years it appeared that M was answerable to the Minister of Defence. In Quantum Of Solace, M was called in to report by the Foreign Secretary - which is how it would be in real life. And having cleared that minor point up - and in spite of the weather 'round here - I really must get out of the house more often!