Best Dua of Forgiveness (Sayyidul Istighfar)

اَللَّهُمَّ أَنْتَ رَبِّي لَا إِلَهَ إِلَّا أَنْتَ خَلَقْتَنِي وَأَنَا عَبْدُكَ وَأَنَا عَلَى عَهْدِكَ وَوَعْدِكَ مَا اسْتَطَعْتُ وَأَعُوْذُ بِكَ مِنْ شَرِّ مَا صَنَعْتُ أَبُؤُ لَكَ بِنِعْمَتِكَ عَلَىَّ وَأَبُؤُ بِذَنْبِي فَاغْفِرْ لِي فَإِنَّهُ لَا يَغْفِرُ الذُّنُوبَ إِلَّا أَنْتَ

Transliteration

Allaahumma 'Anta Rabbee laa 'ilaaha 'illaa 'Anta, khalaqtanee wa 'anaa 'abduka, wa 'anaa 'alaa 'ahdika wa wa'dika mas-tata'tu, 'a'oodhu bika min sharri maa sana'tu, 'aboo-'u laka bini'matika 'alayya, wa 'aboo-'u bidhanbee faghfir lee fa'innahu laa yaghfiruth-dhunooba 'illaa 'Anta

Meaning

O Allah, You are my Lord, none has the right to be worshipped except You, You created me and I am Your servant and I abide to Your covenant and promise as best I can, I take refuge in You from the evil of which I have committed. I acknowledge Your favour upon me and I acknowledge my sin, so forgive me, for verily none can forgive sin except You.

Source: Bukhari: 6306