Attributed to VInce Lombardi, the legendary 20th century Green Bay Packers coach, Jack Dempsey, the 20th century prizefighter, and (most likely) Carl von Clausewitz, 19th century Prussian military theorist.
I have to point out that this quote was said in the context of boxing, and many people take it out of that context and put it in places that are completely different. One of the only places where a good defense is a good offense is in boxing.
While I realize this was asked years ago, the somewhat oblivious replies posted by some prompted me to answer anyway. The original quote was, “The best defense is a good offense.” I stumbled across this page while confirming my own memory due to the quote’s use by a character in a story I’m working on at the moment. The earliest person it is attributed to seems to be Mao Zedong, a Chinese Communist revolutionary. Since Zedong likely had nothing to do with boxing, the person who insisted that the quote originated in that arena and taken out of context might need to tweak that theory.
Hope this helps the next person who is researching the line.