Hadith: The most superior way of seeking forgiveness is for a slave to say: O Allah, You are my Lord. There is no god but You. You created me and I am Your slave, and I remain faithful to Your covenant and Your promise as much as I can. I seek refuge with You from the evil of what I have done. I acknowledge Your favors upon me, and I admit my sins. So forgive me, for indeed none forgives sins except You

عن شداد بن أوس رضي الله عنه عن النبي صلى الله عليه وسلم : "سيد الاستغفار أن يقول العبد: اللهم أنت ربي لا إله إلا أنت خلقتني وأنا عبدك وأنا على عَهْدِكَ وَوَعْدِكَ ما اسْتَطَعْتُ أعوذ بك من شر ما صنعتُ أَبُوءُ لك بنعمتك عليَّ وأَبُوءُ لك بذنبي فَاغْفرْ لي فإنه لا يغفر الذنوب إلا أنت".
[صحيح] - [رواه البخاري]

Shaddād ibn Aws (may Allah be pleased with him) reported that the Prophet (may Allah's peace and blessings be upon him) said: "The most superior way of Istighfār (seeking Allah's forgiveness) is for a slave to say: O Allah, You are my Lord. There is no god but You. You created me and I am Your slave, and I remain faithful to Your covenant and Your promise as much as I can. I seek refuge with You from the evil of what I have done. I acknowledge Your favors upon me, and I admit my sins. So forgive me, for indeed none forgives sins except You."
Sahih/Authentic. - [Al-Bukhari]

Explanation

The Prophet (may Allah's peace and blessings be upon him) tells us that the words of this supplication are superior in such a way that makes them like the master of expressions of seeking forgiveness. In this supplication, the slave says: "O Allah, You are my Lord. There is no god but You. You created me and I am Your slave, and I remain faithful to Your covenant and Your promise as much as I can. I seek refuge with You from the evil of what I have done. I acknowledge Your favors upon me, and I admit my sins. So forgive me, for indeed none forgives sins except You." Thus, the slave firstly acknowledges the Oneness of Allah, and that he abides by his oath to Allah, the Exalted, to believe in Him and obey Him as much as he can, not according to what is actually befitting for Allah, Exalted, or what He deserves, because no matter how much worship the slave performs, he will never be able to perform all that Allah orders him to do, nor will he be able to thank Him for His favors as is due for Him. Then he seeks shelter and refuge with Him, for Allah is the one whose refuge is sought against the evil things which the slave commits. Then the slave voluntarily acknowledges Allah's favors upon him, and then admits his own sins and disobedience. Finally, he supplicates Allah to forgive him by protecting him from his sins and their evil by His pardoning, favor and mercy, since none forgives sins except Him, Glorified and Exalted.