What is the Best way of asking Allaah for Forgiveness?

On the authority of Shaddad ibn Aws (رضي اللّه عنـه), the Prophet (صلّى اللَّهُ عليه وسلّم) said, 
“The chief of prayers for forgiveness is:

اللَّهُمَّ أَنْتَ رَبِّي لّا إِلَهَ إِلَّا أَنْت،

خَلَقْتَنِي وَأَنَا عَبْدُكَ،

وَأَنَا عَلَى عَهْدِكَ وَوَعْدِكَ مَا اسْتَطَعْت،

أَعُوذُ بِكَ مِنْ شَرِّ مَا صَنَعْت،

أَبُوءُ لَكَ بِنِعْمَتِكَ

عَلَيَّ، وَأَبُوءُ بِذَنْبِي فَاغْفِر لِي فَإِنَّهُ لَا يَغْفِرُ

الذُّنُوبَ إِلَّا أَنْتَ

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

The Prophet (صلّى اللَّهُ عليه وسلّم) 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.”