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.
McHale's value goes down if we're playing 2026 basketball, though, which always adds a wrinkle to these sorts of all-timer lists if you're fortunate enough to be spoilt for choice at most spots.
Compared to who? Garnett? Heinsohn? Cowens?
I agree that it is hard because if you are comparing the individual players to others of their era, then Cousy has to be on any list. If you are taking a player from an earlier era and trying to say who would be best for the current era, it is a near impossible to do that in any consistent way (everyone will have a different opinion). But if you stick to the eras, you can say player X was an all star ## of times while player Y only # times, use that kind of thing to support a ranking. You also get in to the question of if it includes accomplishments on other teams or just in Boston, for example when trying to compare Garnett to McHale.
Dennis Johnson seems to be coming up a lot. He was at best, the 4th best player on his Celtic's teams. If DJ is on the list but McHale and Parish not, I don't get it. You could make the argument that he would be a better fit with the other players on this hypothetical Green Dream Team, if this team played in the modern era, but I don't see it. If you go combo guard, you could slot Hondo in that spot. if you go pure PG, it has to be Cousy.