Firstly: The Ruling of the Opening Supplication
The opening supplication is one of the sunnahs of prayer.  One recites the opening supplication for every prayer, whether wajib or additional. This includes reciting it at the start of every two units of prayer during the night prayer. This is the position of the majority: the Hanafis, Shafi`is, and Hanbalis. It is the position of the majority of scholars. Secondly: Some Formulations of the Opening Supplication O Allah! Separate me and my sins as You have separated the East and West. O Allah! Cleanse me of my sins as a white garment is cleansed of dirt. O Allah! Wash my sins with water, ice, and hail. All praise is due to Allah, a praise that is abundant, pure, and full of blessings. Allah is altogether great. Praise is due to Allah in abundance. Glory be to Allah in the morning and evening. Glory and praise be to You, O Allah. Blessed is Your name and exalted is Your majesty. There is no god but You. Thirdly: The Preferred Formulation for the Opening Supplication
It is best that one follow all opening supplications that have been narrated from and authenticated to the Messenger of Allah ﷺ, varying the opening supplication in prayer. Ibn Taymiyyah chose this position, as did al-Sa`di, Ibn Baz, al-Albani, and Ibn `Uthaymin.