If it's just x's and o's then I'd have to say it was a tie between Fitch and Heinsohn, but overall I'd give it to Heinsohn because I think he got more out of less talent than Fitch did and his players didn't eventually tune him out.
But my memory only goes as far back as the 70's Hondo days so I can't really comment on Russell's coaching other than that of what I read and that was that he pretty much just continued the Auerbach system.
Worst, well that's a different story. Recently we've had some really bad coaching.
Carr was atrocious but he was trying to be atrocious, so I'm not sure he can be properly judged. He so blatantly tried to lose that season I'm surprised he wasn't sanctioned by the league. Of course, he didn't end up with Duncan, so maybe he was sanctioned by the league.
Pitino was bad and I don't buy the whole, he was a decent coach but a horrible GM argument. he brought in players and got rid of players based on a style of play he wanted to play. That style, end to end defense and college level offense wasn't suited for this league and because of that he failed as a coach. his handling of the players was about as bad as has ever occurred here and sorry people, that is a part of coaching.
Doc had a couple of dreadful years here as well. Again, two years ago he was obviously tanking and not surprisingly the Celtics once again got screwed in the draft for being so blatant. I'm not saying it was the league that fixed it but sometimes fate rewards and punishes such obvious misdeeds. But as good as Doc coached last year, and I do think he had an exceptional year as a coach, he was just as bad in other years.
But without a doubt the worst Celtic coach ever was John Carroll. He had zero skills and zero control over that bunch after O'Brien was let go/left. That team went from under-performing to catastrophically awful just like that. I know it was just a stop gap but those stop gap guys usually show a little something and get the team to respond if only because there's been a change. Not that group. They quit and it was immediate.