Ainge has certainly done well at the top of the draft, even when that's been harder than it sounds. Jaylen Brown wasn't necessarily obvious guy to choose at #3 in 2016 (there were groans at the Celtics draft event when the pick was announced), and Markelle Fultz was the obvious guy to choose at #1 to many. Looking at players drafted at the top of those 2 drafts, Ainge went 2 for 2 when that outcome was by no means assured.
Bk-Ref has,
in that draft, that Jaylen Brown is ranked 7th by WS, 15th by WS/48, 14th by BPM and 12th by VORP. Simmons ranks first in 3 of the 4, though he obviously wasn't available at #3. But other top picks including Sabonis, Jamal Murray, Poltl and Buddy Hield all rank above him in terms of results. That's before you get to deeper-round picks that happened to pan out, like Siakam, which I don't think is fair to hold against him.
I'm an NBA advanced-stat newb, but other than avoiding Dragan Bender I'm not sure Ainge really hit a bullseye there. He scored highly, he avoided a big miss, but didn't max out his value, either. And before anyone confuses what I'm saying: I love Jaylen Brown and love watching him. I just don't want to overstate the case for Ainge there. He did OK in 2016 (and probably hit median-to-above-average expectation with Rozier in 2015 too), but it was 2017 where he really shined. Likewise 2014 (Smart), unless you want to hold "not drafting Jokic" against him, along with holding that against the rest of the league.
I think the important conclusion we can take from a long career of Ainge decisions is that:
(1) He
doesn't fuck up the important decisions. The biggest picks you can say were a bust relative to their slot were probably Yabusele (2016/#16), James Young (2014/#17) and Fab Melo (2012/#22). Everything else, including every pick higher than that, has panned out to a greater or lesser degree.
(2) He has some of that Auerbachian ability to win trades that make people shake their heads later.
That alone makes him a top 5 GM in the league.