Cousy, Russell, Havlicek, Cowens, Bird, and Pierce
That would be the guys whose number I think should clearly be retired.
I'd listen to arguments for Sam Jones, Parish, McHale, and Lewis.
Anyone else cheapens it for the rest of the players. Just my opinion, but the ultimate honor a team can bestow on a player is having his number retired. There should absolutely be some level of games played, statistical feats, and winning. If KG was the anchor (or a main player) on 3 or 4 title teams in his 6 seasons in Boston, then I think you overlook the fact that it was just 6 mostly injured seasons, but the team won just 1 title and he played just 396 regular season games and just 84 playoff games (missing a post season entirely). That shouldn't be the standard for getting your number retired. it just isn't enough.