A writer for Tom Ricks' blog draws attention to a recent interview by former Secretary of Defense Bob Gates praising two of his predecessors, Mel Laird [under Nixon] and Harold Brown [under Carter]. I share Gates' admiration and would add Bill Perry, Brown's deputy and later Clinton's second SecDef.
My own study of those who have held that office, SecDef: the nearly impossible job of Secretary of Defense, was published just before Gates took office in 2006. I praised Laird, Brown, and Perry as unsung heroes who performed all of their key tasks with great skill, and did not join the ranks of 1/3 of the Pentagon leaders who were fired or forced to resign. In addition to managing the department, the secretary has to maintain good relations with the President and other members of the National Security Council, with the military leadership, and with the Congress. He also functions as a war planner and an important diplomat.
Gates himself would rank first in my judgment. He maintained the confidence of two quite different presidents and kept strong congressional support, despite his own contempt for politicians, which he revealed only in his memoir. He also made many good calls -- in the wars in Iraq and Afghanistan, in reorienting military thinking to the most urgent tasks, and in demanding accountability. He even fired people, which few of his predecessors had ever done.