We've had some poor Defence Secretaries under the present administration but Bungling Bob is the worst.
Two other contenders to remind you of are the inept Des Browne who did it as a job share whilst also being Scottish Secretary and John Reid who famously claimed that British troops would leave Afghanistan "without firing a single shot" (Reid has a nice Directorship with privatised prison operator G4S - was this anything to do with his spell as Home Secretary ? )