Ainge has a very good track record of getting among the best available players with his 1st round picks
Just going by winshares, looking at the players that could have been drafted instead:
2019: Grant Williams- Only 4 players taken after him the rest of the draft have more WS, yeah he's not great but there was no great to take, Langford it's way too early to tell anything given injuries
2018: Rob Williams- Only 2 players taken after him with more WS, Brunson and Robinson, I'm sure none of us would trade TL for Brunson, Robinson is debatable but at worst he got the 2nd best player available to date, home run
2017: Tatum- You could debate if he's the best player in the whole draft, but no matter what home run pick
2016: Yabu sucked, Zizic wasn't much better, but we all know why they were drafted, so kind of need to ignore this one because Ainge's hands were tied, and he hit a home run with Brown
2015: Only a handful of guys taken after Rozier ended up better, 2 in the rest of first round. Hunter sucked, but no stars to even miss on
2014: Young sucked, but hitting on Smart cancelled that out some
2013: Only 3 guys taken the rest of the draft after Olynyk were better, yeah 2 were superstars, but hard to argue for getting this pick wrong
2012: Melo sucked, Sullinger was one of the best guys available, and the best taken in the rest of the round
You can go back further but it's largely the same, maybe he didn't often draft who ended up being the best or 2nd best guy available, but he usually drafted one of the handful of best available, and he has 2 mid/late 1sts that vastly exceeded expectations, and a 3rd that is hopefully on the way.