[Some Rulings on Reciting the Qur’an and Remembering Allah Most High 4]

Answer: The best supplication is the supplication found in the Opening of the Book: “Guide us to the Straight Path—the path of those upon whom You have bestowed favor, not of those who incur wrath nor of those who go astray,” because Allah Most High has made it obligatory to invoke with it in every prayer, and He has not made any other supplication obligatory upon those legally responsible within the prayer. Undoubtedly, what is obligatory in supplication is superior to what is merely recommended.
If it is said: It has been narrated that seeking forgiveness (istighfār) is the best supplication.
We reply: Yes, seeking forgiveness is the best supplication except for the supplication we mentioned in al-Fātiḥah.
Source : Min Thimār al-ʿIlm wa al-Ḥikmah vol.1