Sayyidul Istighfar (Best Dua for Forgiveness)
اللَّهُمَّ أَنْتَ رَبِّي لَا إِلَهَ إِلَّا أَنْتَ خَلَقْتَنِي وَأَنَا عَبْدُكَ وَأَنَا عَلَى عَهْدِكَ وَوَعْدِكَ مَا اسْتَطَعْتُ أَعُوذُ بِكَ مِنْ شَرِّ مَا صَنَعْتُ أَبُوءُ لَكَ بِنِعْمَتِكَ عَلَيَّ وَأَبُوءُ بِذَنْبِي فَاغْفِرْ لِي فَإِنَّهُ لَا يَغْفِرُ الذُّنُوبَ إِلَّا أَنْتَ
O Allah, You are my Lord. There is no deity but You. You created me and I am Your servant. I abide by Your covenant and promise as best I can. I seek refuge in You from the evil of what I have done. I acknowledge Your favour upon me and I confess my sin, so forgive me, for none forgives sins but You.
Sahih al-Bukhari 6306