Robin Lopez with out a doubt IMO. The guys has crazy potential. Brook is naturally more skilled offensively, but he doesn't have anywhere near the passion for the game that Robin has, which is why I think Robin will thrive under Monty. Monty will push and give Robin every tool he needs to be a good player, and I think he takes it and runs with it because he loves the game.
My other reason why Robin breaks out is that he looked like a steal in 08 and 09 (before his injury). After his injury, he lost some of his athleticism that made him such a promising young player. Phoenix forced him to play through it, and it caused him to have a bad couple of years. Now, however, he says that he has gained his athleticism back over the past few months after having a surgery on the knee. If he really did gain it back, he could be a great player for us.
This is him pre-injury:
And this is him post-injury:
You can see in the first video (pre-injury) that he possesses insane athleticism for his size, but then you see the 2nd video (post-injury), and you see that he lost his athleticism. He could really be something if he can get back to that. Also, Monty and Ryan Anderson hinted that Lopez is more offensively skilled than people think. We'll see, but if he can give us a back to the basket game, he and Davis could do some serious damage.