Julian - my dad had prostate cancer and I spent significant time and effort trying to help him, over the years, with various aspects of his treatment, so I can, in some way, relate to what you are trying to do.
With respect to "the best" surgeon, I don't think there is such a thing. Or, if there is, I don't think there is any way someone can determine who that person is. (Think of it this way -- who is "the best" philosopher in the United States, or "the best" chef, or "the best" architect). Instead, I think what you want is a surgeon who is very experienced, with a credible hospital, has a reputation for excellence, who your father can trust, and who is not too geographically inconvenient. If you find a surgeon who is affiliated with one of the leading teaching hospitals in the United States, and has done more than, say, 1000 prostate surgeries, and still does at least a few of them every week, that would probably be a reasonable place to start -- IF your dad has decided that surgery is his preferred course of action (rather than, say, radiation).
I don't think you should necessarily limit your search to "robotic" surgery. Indeed, some of the most prominent surgeons in the country do not do robotic surgery. If your dad does choose robotic surgery, definitely find a doc who has done LOTS of them. There's a real learning curve.
One other "helping dad" point I will make -- all this stuff needs, ultimately, to be his decision. It is more difficult than it might seem to be helpful without "telling someone what to do." Through the course of his treatments, my dad made some decisions that I disagree with. I still think I was right. But that is OK -- it was his body and his illness and his life, and he made the decisions that he wanted to make.
Best wishes, Medved