How about you take away Russell and Bird?
otherwise this thread will just go back and forth forever...
Taking away those two, it's a tougher call. It comes down to three in my mind:
Bob Cousy would have been a great choice. A point guard that controls every game he played, running a transition offense to perfection.
Dave Cowens is a name that no one has brought up yet. An outstanding hustle big man who outworked Reed, Kareem, and the like and could pass well. Going by the successful Patriot standard of versatility as well, Cowens could play the 3, 4 or at times the 5 to go small and run the floor with his passing skill (mostly the three and four in the modern game, as 6'8, 230 wouldn't do much against a 7'1 320 lb Shaq). Cowens is worth consideration.
However, without Russell and Bird, the answer for me is KG. Anyone that can inspire that defensive commitment in an entire team is the guy I want to build my team around. Match him up with a bruiser in the post (like Perk), and a point guard that can create for the team and KG will get his shots. KG makes everyone around him better defensively, because he frankly won't allow anything less in practice. He's one of the best peer motivators in Celtics history by all accounts, and the guy I'd build with if Russell and Bird were not available.