Now for the 1st dumb answer of the thread:
My vote is "Ultra" Not a weapon but the Germans didn't see it comming that's for sure!!
In the years running up to the war Poland had worked on various techniques for cracking Enigma. Shortly before the German invasion of Poland, they shared their work with their British allies. Poland's government was the first to employ mathematicians as code-breakers, and the mathematicians' logical minds proved to be just what was needed to tackle Enigma.
That 1 "weapon" or secret shortened the war by at least 2 years..and could of been the turning point, or the deciding factor in winning the war.