The answer is probably Fox based on his skillset and current landscape of the NBA with guards dominating. He has the ability to not only make teammates better but also change the entire style of the game both offensively and defensively. I'm starting to question his motor just a little too but I think that's mostly lack of strength and he's wearing down. It's also his second year in the league and I'm not sure he feels entirely comfortable yet playing with aggression and truly commanding his teammates. With that said I think it's pretty obvious that Fox doesn't have Buddy's motor (who does?) or Bagley's motor........but in general most starting PG's don't need that kind of motor and need to be more cerebral with a feel for flow of the game.
Bagley is not a franchise cornerstone. No matter how much potential and athleticism he has, he's never going to be true #1 that can get his shot whenever he wants it. More importantly he doesn't create shots for others. He may turn out to have leadership skills but he's not the type of player to "lead an offense". He could very well be super inside scorer and finisher. Best case scenario is the Amare Stoudemire and Chris Bosh comparisons. Prime Amare was the ultimate finisher and almost impossible to stop when paired with a great PG.
Giles is the wild-card and I don't think any of us (including Giles) himself really know what Giles potential/ceiling is yet. Giles natural skills and basketball IQ are incredible (especially for a 20 year old big man). Bagley is a better athlete with the higher motor but no question Giles is a smarter player that you can build an offense around and run the offense through. He can absolutely make teammates better and a guy like Bagley could really benefit from having two elite playmakers creating easy shots for him.
I don't think Giles is a natural high volume scorer though. The Webber comparisons are pretty accurate in terms of body, hands, and skills but I don't think Giles will ever be quite as comfortable being a 20-25 PPG scorer. He really likes to pass which will hamper his own offense and I don't think he's a natural aggressive finisher like Bagley is. Even when he's playing well Giles still comes off as a bit too robotic to me as well, he doesn't just get the ball and go like true scorers do. Giles could very easily be a 16-19 PPG scorer though and when you combine that with Fox, Bagley, and Buddy that should be more than enough. As usual the only big question with Giles is whether his knees are going to hold up and all we can do is hope for the best.