No GM is perfect, and Danny has certainly made some mistakes. Some of those seemed clear at the time, while others looked like solid moves that just didn't pan out.
If you could change just one move, which would it be? The only caveat is that the move has to be based in reality; there's no "I would have signed Lebron to the MLE" allowed.
With the benefit of hindsight, I'll say that the top move I'd change is drafting Gabe Pruitt over Marc Gasol in 2007. That pick would have given us the center we've needed these past several seasons, and arguably would have kept Pau Gasol away from the Lakers.
My other big "what if" that isn't necessarily obvious to most Celtics fans is, what if Danny had included top-35 protection on the second rounder he sent to Minnesota in the Mark Blount / Wally deal. That pick turned out to be the 31st pick in the 2008 draft, which turned into Nikola Pekovic. That's a move that you can't criticize Danny for, but it's interesting to Ponder what could have happened.
And last, there's the blatantly obvious "what if Danny had drafted a competent player over J.R. Giddens in 2008", with the most likely candidate being DeAndre Jordan.
That's three starting centers that we could have had, all available with just a little more luck and/or foresight.