I wouldn't want to be the writer, unless I was adapting my own book, because they don't get enough credit for being key to the whole thing. I wouldn't want to be the director - too much responsibility. I wouldn't want to be the producer because they spend most of the time tearing their hair out. I wouldn't want to be an actor, because I would worry that I would be rubbish.
What I would really really love to do, is make the "Making of" Documentary (Write, direct and produce that). Far more manageable, and less waiting around, but enormously satisfying to work on itself.