I guess there are two ways you could go with this:
1) Guys who were most effective as Celtics, but failed to win a title while here.
2) Guys who either were good here and never won, or were great elsewhere and but didn't win it all here.
List one is fairly short, but must include Antoine, Dee Brown, and well...jeez that's gonna be a short list...
2nd list will be more fun:
Shaq, Sheed, Billups, Toine, Artis Gilmore, Pete Maravich, Dominique Wilkins...any other ideas?
...yes, it's the off season