Ulum al Qur'an
Grades of Sources [See Ibn Taimiya, op.cit., p.93.]
The best Tafsir is the explanation of the Qur'an by the Qur'an.
The next best is the explanation of the Qur'an by the Prophet Muhammad, who, as Shafi'i explained, acted according to what he understood from the Qur'an.
If nothing can be found in the Qur'an nor in the sunna of the Prophet, one turns to the reports from the sahaba. [See Ibn Taimiya, op.cit., p.95.]
If nothing can be found in the Qur'an, the sunna and the reports from the sahaba, one turns to the reports from the tabi'un. [See Ibn Taimiya, op.cit., p. 102.]
However, nothing can match the explanation of the Qur'an by the Qur'an and the explanation of the Qur'an by the Prophet.