The most fitting supplication is for a person to say, "O Allah, You are my Lord and I am Your servant. I have wronged myself and acknowledged my sin. My Lord, forgive me my sin, for You are my Lord, and none forgives sins but You."
حدثنا روح بن عبادة حدثنا شعبة عن يعلى بن عطاء قال سمعت عمرو بن عاصم بن سفيان بن عبد الله قال سمعت
إن أوفق الدعاء أن يقول الرجل اللهم أنت ربي وأنا عبدك ظلمت نفسي واعترفت بذنبي يا رب فاغفر لي ذنبي إنك أنت ربي إنه لا يغفر الذنب إلا أنت
Its isnad is sahih. Bukhari related it 1/211 in the chapter on adhan, on supplication before the taslim, and Muslim 5/84 no. 2709 in the chapter on remembrance, on the recommendation of lowering the voice in remembrance, and Tirmidhi 5/543 no. 3531, saying it is hasan gharib, and Ibn Majah 2/1261 no. 3835, all of them narrating it as a raised hadith.