Best Dua for Seeking Forgiveness

Astaghfirullah Rabbi Min Kullay Zanbiv v Atoobu Alay.

Allahummaghfirli khatāyaya wa dhunubi kullaha(O Allah!  Forgive all my sins and ommissions.)

اللَّهُمَّ اِنّى اسْتَغْفِرُكَ لِذَنْبِى وَ اسْئَلُكَ رَحْمَتَكَ

Allahumma inni astaghfiruka li dhanbi wa as’aluka rahmatak(O Allah!  I ask You for Forgiveness for my sins, and for Your Mercy.)

سُبْحَانَكَ اللَّهُمَّ وَ بِحَمْدِكَ، أشْهَدُ أنْ لا إلهَ إلَّا أنْتَ، أسْتَغْفِرُكَ وَ أتُوبُ إلَيْكَ

Subhanakal-Lahumma wa bihamdik, ash-hadu allaillaha illa ant, astaghfiruka wa’atoobu ilaiyk

(Glory be to You, O Allah, and all Praise, I testify that there is no deity but You, I seek Your Forgiveness and to You do I repent.)
[3 times] [recommended for at the end of meetings, atoning for infractions during it]
[at Tabari, an Nasa'i, al Hakim]

رَبَّنََآ إنَّنَآ ءَاَمنَّا فَاغْفِرْ لَنَا ذُنُوبَنَا وَ قِنَا عَذَابَ النَّارِ

Rabbana innanā amanna, faghfir lana, dhunoobana wa-qinna ‘adhāāban-naar

(Our Lord!  we have indeed believed: forgive us, then, our sins, and save us from the agony of the Fire)
[surah Ali' Imran; 3:16]

سُبْحَنَ رَبِّنَآ إنَّا كُنَّا ظَلِمِينَ

Subhāna Rabbina inna kunna dhalimeen(Glory to our Lord!  Verily we have been doing wrong!)
[68:29]

لَّا إِلَهَ إِلَّا أَنتَ سُبْحَانَكَ إِنِّي كُنتُ مِنَ الظَّالِمِينَ

La illaha illa Anta Subhanak, inni kuntu min adh-dhalimeen (There is no God but You;   Glory be to You, Truly I have been one of the wrongdoers)
[Surah al-Anbiyah, 21: 87]

سُبْحَنَكَ تُبْتُ إلَيْكَ وَ أنَاْ أوَّلُ الْمُؤْمِنِينَ

Subhānaka tubtu ilaiyka wa ‘ana awwal-ul mu’mineen(Glory be to Thee!  To Thee I turn in repentance, and I am the first to believe)
[surah al-A'raf, 7:143]

اللَّهُمَّ اعْفِرْلِى ذَنْبِئ كُلَّهُ، و جِلَّهُ، وَ أوَّلَهُ وَ آخِرَهُ عَلا نِيَتَهُ وَ سِرَّهُ

Allahumma-ghfir li dhanbi kullahu, diqqahu wa jillahu, wa awwalahu wa akhirahu, wa ‘ala niyatahu wa sirrahu

(O Allah!  Forgive me all my sins, great and small, the first and the last, those that are apparent and those that are hidden)
[Muslim 1:350]

اللَّهُمَّ إنِّي أسْألُكَ يَا اللَّهُ بِأنَّكَ الْوَاحِدُّ الأحَدُ الصَّمَدُ الَّذِي لَمْيَلِدْ وَ لَمْ يُولَدْ وَلَمْ يَكُنْ لَهُ كُفُواً أحَدٌ، أنْ تَغْفِرَ لِي ذُنُوبِي إنَّكَ أنْتَ الْغَفُورُ الَّرحِيمُ

Allahumma inni asaluka ya Allahu bi annakal Wahidul Ahadus-Samadul ladhi, lam ya-lid wa lam youlad, wa lam yakullahu kufuwan Ahad, an taghfirali dhunoobi innaka Antal Ghafūūrur Raheem

(O Allah!  I ask You, O Allah, You are the One, the Only, Self Sufficient Master, who was not begotten and begets not and non is equal to Him.  Forgive me my sins, surely you are Forgiving, Merciful.)
[Abu Dawud, an Nasa'i, ibn Majah, at Tirmidhi]

أسْتَغْفِرُ اللَّهَ وَ أتُوبُ إلَيْهِ

Astaghfirullaha wa atūūbu ilai(hi)(I seek the Forgiveness of Allah and repent to Him.)