What is the greatest Surah in the Qur’an?

The greatest surah in the Qur'an is Al-Fatihah, because of the Prophet's (peace and blessings be upon him) saying to Abu Sa'id ibn Al-Mu'alla: "I will teach you the greatest surah in the Qur'an before you leave the mosque... (All praise is due to Allah, Lord of the worlds) It is the Seven Oft-Recited Verses and the Great Qur'an which has been given to me."