I think Tatum is the best tookie, but Donovan Mitchell is my sleeper for ROY
I think it's honestly going to be a pretty tough call between Fultz, Ball and Tatum in terms of sheer talent.
Ball is most likely to be a stat sheet stuffer since he has the ability to contribute across a range of categories - I can see him in double figures while also putting up strong assist and rebound numbers. The past players that remind me most of Ball are:
* Ricky Rubio (10.6 PTS / 8.2 AST / 4.2 REB in his rookie year)
* Jason Kidd (11.7 PTS / 7.7 AST / 5.4 REB in his rookie year)
So I wouldn't be at all surprised to see Ball putting up numbers in that range. Something like 12/8/5 I think is quite achievable for him. Though given the team he's playing on (their youth, his probable high usage rate, run and gun style, general roster makeup, etc) I can also imagine his turnover numbers being quite high, and his shooting percentages being quite poor, and his defence being woeful. I can see him shooting something like 38%/33%/67% which (combined with his poor defence) potentially offsets the impressiveness of his other overall numbers.
Fultz is a tough call. With Fultz/Reddick/Saric/Simmons/Embiid the 76ers have a pretty solid young roster, though even with Fultz on board I think Embiid is still their Alpha scorer, and Simmons is still their most talented playmaker, with Fultz being a close second at both categories. Because of this, I think Fultz's numbers will depends a LOT on the health of those two guys. I feel like those three guys all NEED the ball in their hands to be most effective, and if those all stay healthy then I'm not sure how Philly is going to find a way to give all three (Simmons, Embiid and Fultz) the touches they need to perform at their best. There is a risk that Fultz could end up getting the Kevin Love treatment (gets relegated to playing off the ball as a spot up shooter) and if that happens his production could suffer. If either Simmons or Embiid (or both) struggles to stay healthy, then Fultz becomes the defacto #1 option for Philly and will have the ball in his hands a lot - and I expect him to put up pretty big numbers in that scenario. 20 PPG certainly isn't out of the question, and I wouldn't be surprised to see him throw in 5 rebounds and 5 assists to go with it. I think there's enough talent in Philly to get him plenty of good looks, so his percentages will probably be solid - something like 45% / 36% / 70%. With his size (6'4"), weight (195) and length (6'9" WS) he has the same size as Marcus Smart, minus some strength, but seems significantly more athletic...so his defensive potential is high, but I think he'll be restricted by questionable defensive motor and IQ.
Tatum obviously will have to battle for minutes and touches, so he's probably going to struggle to put up numbers competitive with those guys . I can see him putting up Per-36 minutes in the realm of 19 points, 7 rebounds and 3 assists and shooting a relatively efficient 46% / 34% / 81% as a rookie. But I think a simple lack of minutes will hold him back in this scenario given the opportunities that Ball and Fultz will get. I think Tatum Is as good a scoring talent as Fultz is, and I think he'll be the best defender of the three due to versatility (he has the mobility and length to guard 2/3/4) and work ethic. I think he might be better defensively then people expect by the end of the season.
I see Fultz getting ROY in the end because he should put up good stats on a Philly team that should make a pretty significant jump in the W column, and probably makes the playoffs in the horribly weak East this year.