Changed my mind, gonna chime in.
In my opinion, John was most definitely the leader up until Sgt Pepper and so on, when he started to lose interest. Before that I know Paul himself has admitted that he was "bossy"... but not until later on did he start to take over as the "leader" of the group. The thing about that, is it's always bothered me how much flack he got for it, especially during "Let It Be". Paul had to take charge, because nobody else gave a shit anymore. 