Worst - Jason Terry, by far. What a lemon he was. Of course, there's also the White Mamba to consider here
and Ryan Hollins. Ugh.
Best - Definitely Posey (way to let him go, Danny!), followed closely by Eddie House, imo. PJ Brown was huge in game 7 vs the cavs, as we all fondly remember. Even Sam Cassell made meaningful contributions that year. I also thought that Marquis Daniels was an excellent pickup. It's just too bad that he was injured, but that's the chance that you take with Marquis. He sure looked like a world-beater in the first month or so of the 09-10 season.
I was really excited when they signed Terry. I've always enjoyed him as a player. It stunk to keep waiting and waiting for him to show up, and he never really came close.
If Terry would have been signed in addition to Ray Allen, it would have been a good move, imo. As his replacement, though? Absolutely not. I think we all saw the mistake Ainge made in the first game that year, when Ray had 19 points and Terry was horrible. That contest also showed that we had overpaid to get him, especially when Leandro Barbosa, who was signed for a fraction of the cost, had come off the bench to provide the type of production that Terry was acquired for, ie, 16 points in 15:50. I miss Barbosa. He was great. It would have been better to, in this order, resign Ray, sign Barbosa, and then go out and get Rashard Lewis, or, if he's snatched up by the Heat, Antawn Jamison. Now that team is loaded.
But back to Terry. I think it was also Doc's fault, because he never adapted the style of play to his players. Terry needs the ball in his hands to be effective, he's not really a guy who comes off of screens like Ray does. It was a terrible fit, all around.
As for JO, I foolishly wanted him, but I wish that he had been required to undergo the same procedure that Kobe had done before we signed him. It's interesting, and frustrating, to see how well O'Neal can still play, and again, I think that that's on Doc. He treated the guy like Perk on offense, and that's more than a little insulting. Here you've got a guy who actually likes and wants to play inside, and nothing ever became of it. He should have had the wrist surgery, though.
Also, I can't believe that I forgot about Pietrus

He was great, but once again, we finally have James Posey's replacement, and Danny let's him walk. I know that he didn't play much after that, but still. That guy showed up in the big moments on both ends, especially in game 5 against Miami. Sigh.
I know that no one will agree with me on this, but the guy that I hoped Danny would resign was Marbury. Why give Nate Robinson that chance and not him? He would have been the perfect 3rd guard, imo, like Vinnie Johnson was for the Pistons. At the bare minimum, Marbury showed that he could wake up, walk off the bus and run a team haha. His passing was great, and I think that, with a few minor moves, that 09-10 bench could have been much better than the one in 08, and that's saying a lot.