I don't know if he is the "worst ever" as Lincoln's (War Secretaries) Cameron and Stanton, FDR's Woodring, and Kennedy's McNamara probably take the cake...but, according to the rumblings of many high-ranking officers as reported in the MSM, Rummy is pretty well hated at the Pentagon for his management style. The best SECDEFs, IMHO, were Forrestal (the 1st), Henry Stimpson, and the recently departed Cap Weinberger.
"but, according to the rumblings of many high-ranking officers as reported in the MSM,"
Nothing said after this makes any diference.