Sayyidul Istighfar (One time)-

اَللَّهُمَّ أَنْتَ رَبِّي لَا إِلَهَ إِلَّا أَنْتَ خَلَقْتَنِي وَأَنَا عَبْدُكَ وَأَنَا عَلَى عَهْدِكَ وَوَعْدِكَ مَا اسْتَطَعْتُ أَعُوْذُ بِكَ مِنْ شَرِّ مَا صَنَعْتُ أَبُوءُ لَكَ بِنِعْمَتِكَ عَلَيَّ وَأَبُوءُ (لَكَ) بِذَنْبِي فَاغْفِرْ لِي فَإِنَّهُ لَا يَغْفِرُ الذُّنُوبَ إِلَّا أَنْتَ

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 (laka) bithanbee faghfir lee fa'innahu laa yaghfiruth-thunooba 'illaa 'Anta.

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. Narrated Shaddad bin Aus, The Prophet (ﷺ) said: "The most superior way of asking for forgiveness from Allah is (The dua is mentioned above). The Prophet (ﷺ) added. "If somebody recites it during the day with firm faith in it, and dies on the same day before the evening, he will be from the people of Paradise; and if somebody recites it at night with firm faith in it, and dies before the morning, he will be from the people of Paradise."

Reference: Bukhari: 6306