Should you write what you like or what will sell? I find that among my works, I can't say which is better or best; I love whichever one I'm working on right now. I can't imagine being told what to write, and I guess that taking the creativity out of a person's work is what makes a writer a hack. Although I understand the need for editorial intervention once I'm done, before that point I have to write my story. It is this that makes me reluctant to join critique groups that exchange chapters.
I heard a famous author say once that he felt the same way, that submitting his work to others before it was finished would take the story out of his hands. It would be the same to try to write what the market wants right now, not my work. It may sound arrogant, but what I write is mine. I'll tell it the way I want to.