A'uudhu Billaahi minash-shaytaanir Rajeem
Bismillaahir Rahmaanir Raheem
As salaamu'alaikum waRahmatullah waBarakaatuhu!!
What is...
The greatest of favors?
The greatest of gifts?"...it is Allah who has done you a favor, by guiding you to faith..." [Surat al-Hujurat: 17]
So, always repeat to yourself:
"Our Lord, let not our hearts deviate after You have guided us..." [Surat Ali 'Imran: 8]
and
"...let us die as Muslims [in submission to You]." [Surat al-A'raf: 126]
The greatest favour and gift from 'Azzawajal is His guidance. In the Qur'aan, the first du'a is to ask Allah(SWT) for guidance to the straight path, the path of those whom He has favoured, not the path of those who have angered Him nor of those who have gone astray. This is mentioned in the Suurah Faatiha also known as Ummul Qur'aan.
As salaamu'alaikum waRahmatullah waBarakaatuhu:)