I can think of three different answers for this: (1) Eric Clapton, because he was the most well-known of the three musicians (2) Bassist Jack Bruce, because he sang lead vocals on a majority of the group's songs (3) But the one I think is most correct: they didn't have one – I've read that they approached the group as a democracy, with no true leader to help steer them in the direction they wanted to go; they simply followed the flow of what other bands were doing.