Has to be Ayton the only other guys in the conversation IMO are Bagley and Bamba and neither of them is so clearly going to be better that you can pass on Ayton. Even if Ayton never improves at all, which seems unlikely given his age and how relatively new he is to the game, he's already a starting caliber 5 for 10 years in the NBA.
With Bagley the gating factor is his outside shot, his mechanics are pretty good but he's been inconsistent because honestly he never needed it in HS or AAU so he shot it for fun more than anything. If some shot doctor can get him to where he's a good outside shooter, which I honestly think WILL happen, he might wind up better than Ayton. He is MUCH quicker, he plays harder IMO, he's a terrific rebounder in his own right, has the potential to be a very good passer, has good hands, but he's not strong enough to dominate the paint in the NBA the way he did in college and HS so he'll need that outside shot. Ayton is strong enough, just needs coaching he's very limited in low post moves right now, and unlike Bagley, he's already a good outside shooter.
