Just shows how subjective "worst" can be. Best Defense was the bomb (ha!) that snapped the wheels right off the Eddie Murphy "guaranteed hit" wagon. He had three megahits in a row, and then... that. Murphy's entire role was added to an existing movie just to try and salvage it (hence "Strategic Guest Star"), and Murphy himself later admitted the script was awful but the paycheck was amazing.
It was also another setback for Dudley Moore, whose track record was already in decline after being a huge box office star himself a few years earlier thanks to 10 and Arthur.
But on the bright side - even with this turkey on their resume, the writer and director got to make a big-budget flick just two years later... Howard the Duck. 
It's pretty much forgotten now, but at the time it was common to reference "Best Defense" the way we would later reference Ishtar or Waterworld... or Howard the Duck.
btw, one of the two Arab guys in the tank with Murphy in that movie was played by Chris Mahar. Right around that time I worked on a stage play he produced and then house-sat for him while he was on vacation. Cool story bro.