I say that the moment the Borg lost their teeth for me was Dark Frontier. On its own, a very impressive episode. As a part of the overall Trek mythos, it took the menace of the Borg away. A small scoutship manages to walk onto the Borg's front doorstep and fly away unscathed. That should never have happened. The moment the Voyager crew made it to the unicomplex was the moment that the Borg lost their fangs. This is supposed to be the heart of Borg space, the seat of their 'power,' for lack of a better word. And the Voyager crew manage to make a fool of the Collective and walk away.
THAT was when they lost their ability to be a terrifying villain, at least in my book.