Although no longer alive, Simo Häyhä [http://en.wikipedia.org/wiki/Simo_H%C3%A4yh%C3%A4] was one. He killed 800 enemy soldiers, by himself, with only weapons he was able to carry around. It should be noted that he was under attack by tanks, airships and carpet bombing runs during his little spree.
Heinrich Severloh [http://en.wikipedia.org/wiki/Heinrich_Severloh] ties with him, having killed an estimated 2000 on D-day. Unlike Simo, however, he had better equipment and a better defense position, along with fire support.