Flyguy is coming from the perspective, which I agree with, that he has the most raw skill & ability of the three. As Jojo said, he's been watching him heavily since his years with the Hitmen, and there are only a couple people whose opinions I would trust as much as him when it comes to prospects.
If you're asking what makes any of us agree with that, I'd look at it from basic talents and attributes:
Sanheim is easily the best skater of the three, although that's not a knock to either of the others, who are very good skaters in their own right. Sanheim is just at a different level. His offensive instincts are pretty comparable to Ghost where they know exactly when to join the rush, how to get to dangerous areas and have great vision. I'd say Sanheim is better on the rush and a more dangerous passer, though. In terms of a shot, he may be the weakest, although it's not too much of a difference between any of them with how sporadic Ghost's shot has been the last year and a half. In terms of defensive skillsets, Sanheim obviously has the greatest advantage in size and reach and does well preventing zone entries. Notably, he's very good at getting the puck out of the zone and starting the play going back the other way.
This isn't a knock at any of the others, who are clearly more proven than Sanheim, but I think it's very fair to say he has the most talent of the three and has the potential to be the best if developed properly.
