Terry Rozier
I hated this pick when it was made and since then I've only grown to hate it more. Picking James Young is at least defensible in the sense that Ainge swung for a fences and missed. That's fine: you'll never get Giannis if you don't suffer through a few Fab Melos and James Youngs. Rozier was a projected second-rounder that nobody thought had a particularly high ceiling that we took a reach on based on... God knows what.
What's worse is that he plays a position where we already had massive redundancy, and this is particularly glaring when you consider that there were multiple serviceable players on the board who addressed our biggest need (wing depth.) RHJ is the obvious miss, but i find it hard to believe that Sam Dekker or Justin Anderson wouldn't be taking at least some of the "Jerebko at SF" or "Smart at SF" minutes if we had got them.
To be fair, the fact that he plays the same position as our best player, best prospect, and best sixth man means that he hasn't gotten as much chance to prove himself as Young or Hunter have. I don't think I've ever actually seen him playing non-garbage minutes. But it's concerning to me that he can't even get some Phil Pressey minutes when we consider that even Phil Pressey could get Phil Pressey minutes. Hell, we waived Phil Pressey because this guy was on the roster - in that sense, there's been an actual downgrade. We no longer have a quick-footed third PG who can come on and push the pace for 3~5 minutes when the opposition is tired.
Edit: Just noticed your updated post about Rozier, you you can probably ignore much of the below haha

You are so very wrong about Rozier.
I'm going to put this out there right now - Rozier has more upside then Smart. You're going to say I'm crazy and that I'm living in a dream world, but come back here 2-3 years from now and we will see who is the better player.
Rozier is a two-way guard with elite athleticism who can score in a variety of ways. He can get to the basket almost at will, he can score from midrange and he can hit the three. He's an exceptional rebounder for a small guard (think Rondo/Westbrook like) and he's actually a pretty solid ball handler - FAR better than Smart is.
To add to that he is an exceptional defensive prospect, and was probably one of the top 2 or 3 defensive guards in the draft.
Then to add to all of that, he has a winners mentality and is a hardcore competitor with elite work ethic (much like Smart).
Smart is an exceptional defensive player who has great size, but his quickness is a little bit limiting and he might never develop into an above average offensive player - I can't see him ever averaging more than maybe 14-15 PPG.
Rozier has the talent to be a 20 PPG scorer in this league.
Imagine Isaiah Thomas' offensive skillset with Avery Bradley's size and defensive skill set and you basically have Rozier in a nutshell.
His potential is sky high, and he really doesn't have a major weakness to his game - there isn't really any aspect of his game where he isn't at least decent.
* In College Rozier was a 17 PPG scorer in his final year with a +18 net rating (108.3 OffRtg, 89.6 DefRtg).
* In the D League he is averaging 19.4 points, 8.0 assists, 6.4 rebounds, 1.9 steals
* Today against Toronto he had 7 points, 3 rebounds, 5 assists and 0 turnovers in 10 minutes on 3-4 shooting, so he's already starting to show SOME flashes of what he is capable of despite getting barely any opportunity
Brad Stevens has already gone on record saying that Rozier has been playing great in practice and has shown that he is ready to play on the NBA stage.
Don't underestimate this kid, he is going to be a hell of a player.