Ooooh, this is such a good question!
On a personal level, I would like for my boss to not only believe in it but really understand how it feels. I'm sick of being asked: "Why can't you do this task? We can pay you for your time if you go over your hours."
And although my PCP believes that ME/CFS exists, I don't know how seriously she takes it and whether she truly understands the impact it has on all aspects of my life, so I'd add her to the list.
On a political level, I would love for a head of state (president, prime minister, etc.) to really believe in it and care. Here in the United States we're working on the head of the NIH, Dr. Collins, to believe in ME/CFS and take it seriously so researchers can get more funding. I would also love for the executive of a major insurance company to believe in ME/CFS and make sure that we get the coverage we need.