In their prime ... I think it would be Shaq, actually. A Celtic, I want to say KG, but I'd actually go with Rondo ... I think it's been shown, at least in an "on-the-surface" kind of way, that KG is more successful as an addition, and not as someone to build a team around, (Ray and Paul, as well).
Rondo took on a leading role in his second year, (first year as a starter), and led it to the Championship, and I think this kid has just begun to tap into his talents. He has shown an amazing ability to adapt and handle enormous pressure, all the while improving by leaps-and-bounds in front of our eyes.
Yup, I go with Rage.