10. The Silence
Personally, I have liked every episode that has featured the Silence. They have played such an important role in Smith's era of Doctor Who to not be loved. But despite my love for them, their time on Doctor Who will need to end soon. Although they only featured in a few episodes, they were mentioned many times from Series 5 onwards. "Silence will fall when the question is asked". And of course, this links in with the Fall of the Eleventh, and it seems clear now that the Fall of the Eleventh will occur at Christmas. We may even see them appear in the 50th as the question they refer to is "Doctor Who?". And with the recent revelation of John Hurt being some sort of forgotten Doctor, he is Doctor Who. So it seems they will fight the Doctor one last time. Whether it be for the 50th or Christmas. When the mystery of the Silence has been solved, it will be the right time to say goodbye. They will no longer have a reason to be in Doctor Who. And I fear that if over-used, it will scare away their initial fright factor.