Amir Johnson is my #1, although I hope they could get him for closer to $3-4 million per year. He is hitting that age that a lot of big men really start to mature at (he will be 23 in May), and I think he could really benefit from learning from Clifford Ray and Garnett, as well as playing with Rondo, who can take advantage of his athleticism.
He also was an Ainge guy when he was drafted, and with Perk's injury, and the possibility of Sheed leaving, he would fill a major need.