Best Dua for Seeking Forgiveness (Syed-ul-Istighfar)

Shaddad bin Aus (Radhi Allah Anh) said: Prophet Muhammad (Sal Allahu Alayhi Wa Sallam) said, “The best supplication for seeking forgiveness (Syed-ul-Istighfar) is to say:  

اَللّٰهُمَّ اَنْتَ رَبِّيْ ، لَا اِلٰهَ اِلَّا اَنْتَ خَلَقْتَنِيْ وَاَنَا عَبْدُكَ ، وَاَنَا عَلَی عَهْدِكَ وَوَعْدِكَ مَا اسْتَطَعْتُ اَعُوْذُ بِكَ مِنْ شَرِّ مَا صَنَعْتُ ، اَبُوْءُ لَكَ بِنِعْمَتِكَ عَلَيَّ ، وَ اَبُوْءُ بِذَنْبِي ، فَاغْفِرْ لِيْ ، فَاِنَّهُ لَا يَغْفِرُ الذُّنُوْبَ اِلَّا اَنْتَ

Allâhumma Anta Rabbî, lâ ilâha illâ Anta, khalaqtanî wa anâ 'abduka, wa anâ 'alâ 'ahdika wa wa'dika mastata'tu, a'ûdhu bika min sharri mâ sana'tu, abû'u laka bini'maktika 'alayya, wa abû'u bidhanbî, faghfir lî, fa innahu lâ yaghfirudh-dhunûba illâ Anta

O Allah! You are my Rabb. There is no true god except You. You have created me, and I am Your slave, and I hold to Your Covenant as far as I can. I seek refuge in You from the evil of what I have done. I acknowledge the favors that You have bestowed upon me, and I confess my sins. Pardon me, for none but You has the power to pardon

Whoever makes this dua during the day with firm belief in it and dies on the same day (before the evening) he will be one of the dwellers of Jannah (Paradise); and whoever makes this dua during the night with firm belief in it and dies before the morning, he will be one of the dwellers of Jannah (Paradise).”

[Al-Bukhari]