The best doa of the Day of Arafah is: 'La ilaha illallahu wahdahu la sharika lah, lahul-mulku wa lahul-hamdu wa huwa 'ala kulli shay'in qadir.' Nabi said this is the best doa and the best of what he and all prophets before him have said. On Arafah, Allah frees more people from the Fire than any other day.
لَا إِلَٰهَ إِلَّا اللَّهُ وَحْدَهُ لَا شَرِيكَ لَهُ، لَهُ الْمُلْكُ وَلَهُ الْحَمْدُ وَهُوَ عَلَى كُلِّ شَيْءٍ قَدِيرٌ
La ilaha illallahu wahdahu la sharika lah, lahul-mulku wa lahul-hamdu wa huwa 'ala kulli shay'in qadir.
Meaning: There is no god but Allah alone, with no partner. His is the dominion and His is the praise, and He is able to do all things.
Source: Sunan al-Tirmidhi #3585The Day of Arafah is the greatest day of Haji and the greatest day of the year for doa. Nabi (shallallahu alaihi wa sallam) said: 'There is no day on which Allah frees more people from the Fire than the Day of Arafah. He draws near and then boasts to the angels, saying: What do these people want?' (Sahih Muslim 1348). Spending this day in sincere doa is the single most important act of Haji.
لَا إِلَٰهَ إِلَّا اللَّهُ وَحْدَهُ لَا شَرِيكَ لَهُ، لَهُ الْمُلْكُ وَلَهُ الْحَمْدُ وَهُوَ عَلَى كُلِّ شَيْءٍ قَدِيرٌ
The Prophet (peace be upon him) said: 'The best supplication is the supplication on the Day of Arafah, and the best of what I and the Prophets before me have said is: La ilaha illallahu wahdahu la sharika lah, lahul-mulku wa lahul-hamdu wa huwa 'ala kulli shay'in qadir.'