Booker is a mediocre shooter at just 32.6% from 3 and as result his TS% is just 57%. He turns the ball over a ton as well (and he keeps getting worse at that). He is a good passer and rebounder for his size and position and he has improved his 2PT% a great deal, but his 3 PT% is bad this year and has regressed. Maybe it is a fluke or maybe it isn't. And in 4 years Booker has shown no improvement at all defensively. He hasn't shown the ability to be an even average defender. Booker is vastly becoming an inefficient gunner. he is still young, but there is no way I'd take Booker over Ayton (assuming the exact same contracts). None at all.
I really think Ayton is getting overlooked in this. His rookie year has been absolutely fabulous averaging 16.4 ppg, 10.5 rpg, 1.9 apg, and about a block and steal in just under 31 mpg while shooting 58.8% from the field and 75.7% from the line (61.2 TS%). He has 4.8 win shares on a team that has won only 12 games. He has a positive OBPM and DBPM and a VORP of 1.2. His rookie year compares quite favorably to Towns, yet I think Ayton has far more defensive potential than Towns has shown now in his 4th year. I'd also much rather have Ayton than Booker who wouldn't know what defense was if it hit him in the face. There are only 4 players that have played for the Suns this year whose ORTG is larger than his DRTG - Holmes, Ayton, Chandler, and Terry (chandler and terry played 9 combined games and 109 minutes total). Booker, for example, has an ORTG 11 worse than his DRTG. The Suns are worse per possession when Ayton is on the bench but are actually better when Booker is on the bench (and since they both start that is a reasonable enough comparison).
I probably have Ayton too low, he could really be as high as #7 or so... but 18-19 year old 7 footers who've flashed the ability to score at all 3 levels in the NBA already- and can nail 3's efficiently at volume, rebound & protect the rim passably- are elite prospects in 2019's NBA. That's why I have LM & JJJ higher, but Ayton could easily be better.
I also disagree about Booker. He's one of the best offensive players in the entire NBA, has no discernible offensive weakness and is only 22. I think his D could improve over time and I'm certain his advanced metrics will when his teammates get better. The Suns also stink at developing talent so he probably still has plenty of upside.
Look, I think Ayton could certainly end up better. I've already admitted I could've put him as high as 7th, behind either Booker or Mitchell. But I think those 2 guys are already really close to becoming top 5 SGs in the NBA already by age 22 (Beal, Klay, Oladipo, DeRozan are ahead, and I likely forgot 1-2 guys, think Harden's a PG). When you have someone that can score close to 25 ppg at age 22, that's a really rare thing. You can build an offense around that, and get a few defenders to put around them. They can also pass very well, and rebound.
If this was 10 years ago I'd put Ayton in the top 3-5, but he needs to adapt his game some to reach his potential.
And about Booker:
1) Cherry picking his 3pt% this year is cute, but he shot 36.3% on 5.2 attempts at age 20 and 38.3% on 7.1 attempts at age 21. He also takes insanely difficult shots- deep 3's, Curry-esque 3's off the dribble, etc. If he had Marcus Smart's shot selection I'm quite confident he'd be over 40%. He won the 3 point shoot out 2 years ago and this year he missed the finals by one 3, still hitting a very high amount. He is a legitimate 3 point shooter.
2) His TOs are high and may have gone up every year, but his assist to TO ratio has gone up the past 3 years. They have made him play out of position at PG and he's turning it over more, but putting up 7 assists a game. He takes very difficult shots and has to do a ton for a poorly run team filled with mediocre teenagers (cough Bender). This drags his numbers down. When you watch him play it's clear that he's as close to Kyrie as any 22 year old alive.
3) He doesn't play defense, but it's hard to because he is the offense for the entire Suns team. Put some competent scorers around him to relieve the burden and maybe he can put more effort into D and experience the Kyrie effect. Put some good defenders around him to mask his deficiencies. Either way, he's top 12-13 in the NBA in both scoring & assists at age 22. He's an elite talent among elite talents, and picks #6-10 are highly debatable to begin with.