Ayesha (rali)said that the Prophet(PBUH) said:
"There is no day in which Allah sets free more servants from jahannam than the day of Arafa. Allah(SWT)'s mercy descends, he boasts to the Angels and says,"What do these people want?"
Best dua to he read on the day of Arafa:
لاَ إِلَهَ إِلاَّ اللَّهُ وَحْدَهُ لاَ شَرِيكَ لَهُ، لَهُ الْمُلْكُ، وَلَهُ الْحَمْدُ، وَهُوَ عَلَى كُلِّ شَىْءٍ قَدِيرٌ