For me, it depends on what era. If we're talking about right now, or within the past 10 years, I would say KG...why?
B/c with Boston's history and popularity and everything, I feel with his game and all, that free agents would more likely come to Boston wanting to win rings. If the past was removed from the Celtics team, and say it was a new expansion team ( ex. Baltimore Dragons...) then you couldn't say KG, Pierce, or Ray b/c they all 3 came from teams that built around them but couldn't produce championships...My initial bet would have been Russell but with him and Bird out...Lets say...Pierce.