Given the high standards of the first three transactions, I think it's gotta be one that helped bring a title. So there's two that jump to mind.
My first one would be trading for DJ. Without him, we would not have been the same team in the 80s. Full transaction was Robey and two 1983 second round picks (Rod Foster #28 and Paul Williams #45) for DJ, a 1983 first round pick (Greg Kite #21) and a 1983 3rd round pick (Winifred King #52). So basically Robey for DJ and Kite - that's huge, to me.
http://www.prosportstransactions.com/basketball/Search/SearchResults.php?Player=&Team=Celtics&PlayerMovementChkBx=yes&BeginYear=&BeginMonth=&BeginDay=&EndYear=&EndMonth=&EndDay=&submit=Search&start=700 June 27, 1983.
The second one might be better because of later effects. On August 4, 1978, we made a huge trade with the Clippers (recently relocated from Buffalo). This was the transaction that led someone to contemplate whether the Celtics were really the Clippers and vice versa. When the owners of the Braves/Clippers and Celtics switched franchises, each kept some personal favorite players via a trade. That trade sent Kevin Kunnert, Kermit Washington, Sidney Wicks and the rights to Freeman Williams for Tiny, Marvin Barnes, Billy Knight, a 1981 second round pick and a 1983 second round pick.
Just getting Tiny was good enough, but the trickle down effects of that trade were enormous. For starters, that 1981 second round pick turned out to be Danny Ainge. Six months after the trade, Billy Knight was traded for Rick Robey. And 5 years after the trade, in the summer of 83, the 1983 second round pick (turned out to be Rod Foster) was packaged with another second round pick and Robey to get DJ and Kite in the trade above. So the overall deal amounted to Kunnert, Washington, Wicks and Freeman for 5 years of Tiny and 4 1/2 years of Robey (1 title), plus Danny Ainge, Dennis Johnson, and Greg Kite (2 titles).
http://www.prosportstransactions.com/basketball/Search/SearchResults.php?Player=&Team=Celtics&PlayerMovementChkBx=yes&BeginYear=&BeginMonth=&BeginDay=&EndYear=&EndMonth=&EndDay=&submit=Search&start=600But since I don't think you can put too much into later trades, I'll rate getting DJ and Kite for Robey just above getting Tiny, Knight and Ainge for Wicks, Williams, Washington and Kunnert.
Other trades I liked:
In 1989, 2 second round picks (Zan Tabak & Richard Petruska) for John Bagley - loved that guy at the BC basketball camp.
Bell & Jones for Banks & Perk.
In 2003, Eric Williams, Battie & Kedrick for Ricky Davis, Mihm and 2nd round draft pick that turned into Gomes.
February of 2005, Jiri Welsch for a 2007 first round pick. That pick was eventually Rudy Fernandez, but we traded that pick at the 2006 Draft for the rights to Rajon Rondo.
Trades that could have been great:
Gerald Henderson for first round pick that turned out to be Len Bias.
Montross and pick that turned out to be Samaki Walker for picks that turned out to be Antoine Walker and Ron Mercer - that Mercer pick could have been a little higher, and a lot better.
And while I like Max for Walton, remember that in addition to giving up Max, who was a great player in his own right, we also gave up a first round pick that turned out to be Arvydas Sabonis. Yeah, he was trapped in Europe, but maybe, just maybe, the draw of the Celtics would have brought him stateside much earlier in his career. I still think it was a good trade since Walton was so important for us for a year and helped make up arguably the best team in Celtics and even NBA history, but enough was given up that I can't really say it was one of the Celtics' five best trades since we had so many fleece trades in our history (Russell, McHale & Parish, DJ, Tiny & Ainge).