If I'm Durant I don't want to go join someone else's team. He'd be doing that with Golden State. Lebum got killed for that.
San Antonio less so, but their two best players really play his position, the 4 or big 3. Their future is far from certain. They have a lot of age and wear and tear on that team.
Miami would be out for me. Wade is an injury waiting to happen, Bosh might never play again but will probably keep dragging down their payroll and Whiteside isn't guaranteed back. Miami doesn't have his bird rights. To keep him they are going to have to max him. Does Durant really want to go to a team that has max'd out Whiteside?
Durant has friends in Boston. He seems to appreciate the history of the Celtics. I think he could see himself being a Legend there. It is a bigger market, but not the hassle of New York or LA.
And I don't think staying in OKC is really an option for him. I think he wants to start his legacy somewhere else with a bright immediate and long term outlook. Westbrook is going to LA next year. I don't think Durant wants to wait around for Westbrook to leave. I think he wants to go to a team where it would be "his" team and he'd be the star.