I will answer this question in more than one way.....
Rondo is the best player to attract other "star caliber talent" players. I love a PG that can rebound, but that is not that position's primary job. Rondo's primary job, to help his team get the best available shot and hopeful score, that is a PG's primary job (secondary job is to take care of the ball so his team doesn't turn it over...which Rondo has had troubles with this season and can do a better job so I am not concerned.)
Sully is our best "inside-out" player, the guy can score down low and he can drain the long distant shot. More and more teams have players with bigs that can shoot the long jumper, and for some reason or another, the inside post player is or has been rare. (I do not count Shaq and Dwight dunking over people, as "post players". Sorry, just my opinion.)
Jeff Green is our best player, overall (decent defender, good scorer - which includes getting to the line and making a high % of FTs), also in the sense that "today's" game is much more "tailored made" for the athletic wing or the off guard. I'm not sure why the league changed this way, maybe it was the "Jordan effect" or maybe the NBA knows its younger fanbase really digs the athletic "star" player.
15 to 20 years ago, the big man and the point guards were all the rage. When the game was just starting, the big man dominated the court. (I wasn't born yet so was not watching so correct me if I am wrong).
Last main highlight I want to leave this thread - *The Rock's voice: "It doesn't matter who the best player is......"
Each player on the team NEEDS each other. Rondo will record ZERO assists if his mates fail to take/make shots. Green's game will suffer if his mindframe isn't focused, Rondo's duties as a PG can help settle all Celts not just Green. Sully's game helps Green's game and vice versa. Defenses will have to gameplan who to focus on defending more, Sully in the paint or on the perimeter or Green in transition, on the break, cutting for an opp or posting a smaller guard....etc. etc. etc.