There are different ways of looking at this -- the best five overall? the best one at each of the five positions? etc. -- but I'll just go with the five guys I'd want on the court:
Russell
Bird
McHale
Havlicek
DJ
Russell and Bird seem obvious and, IMO, don't require an explanation.
McHale was a very good defender and had perhaps the greatest and most varied stable of post moves in NBA history.
Havlicek is the greatest scorer in Celtics history.
DJ, I imagine, would get the most opposition for inclusion on this list, which is understandable because he was never a star, but he was a solid point guard who had championship experience before he even got to Boston, helped them win more titles, had a superhuman on-court connection with Bird, and was, according to either Magic or Michael Cooper (can't remember which), the toughest defender Magic/Cooper ever faced, which I think is high praise.