I tend to think that conventional wisdom places Chief as the hero. He is the one who breaks out, embodies McMurphy's ideals, and evolves as the narrative progresses. However, I think that a good case can be made for McMurphy as the hero of the novel. McMurphy is the force of change. He is the one who initiates so much of the fundamental shift in thinking on the part of the inmates. McMurphy would have to be seen as a heroic figure because he stands up to Nurse Ratched, when so many others, including the doctors, do not.
McMurphy is the hero of the novel because he embodies a conception of freedom, dissent, and autonomy. McMurphy represents the spirit of change, the fundamental ability to transform what is into what can be. I think that this is where he has to be seen as a hero. When he dies, we recognize, as Chief does, his importance as one whose legacy lingers. Chief's entire premise of change is one in that owes a large debt to McMurphy, something he himself acknowledges in having to smother him. It is here where I think that McMurphy can be seen as the hero of the novel.