Pruitt hasn't gotten much love at all on these boards. When Bilas said on draft night Pruitt didn't play defense, he didn't know what he was talking about. Pruitt was known for his defense at USC. He's not a great one on one defender, but is someone who plays the passing lanes. He's got a nice shot, and has a decent handle. He's a tweener and is more or less Delonte West with more athleticism, and not as fragile. He's a team player, and a good guy to have in a back-up role. He'll never be a superstar, but he should, if given a chance, contribute to the team.
BBD is what, 6'6" though listed three inches taller. He'll never be more than a back-up in this league, but he did have a pretty good rookie year.
As far as ranking however, without two of the guys even playing in the league, at this moment they are BBD, Gabe, JR, and Walker. Potentially however, in five years having a rotation of Rondo, Pruitt, JR, and Walker (in whatever order) looks pretty good, and reliable. Of course, the line-up will be much different by then. Nevertheless, I'd say JR has the most upside, then Walker, then Pruitt, then BBD.
Really though, ranking depends on the needs of the team, injuries to the team and to the four individual players, as well as individual characteristics. There's no doubt in my mind that if Tony had not blown out both knees he'd be a far more developed player, and would have been resigned.