The best dua of the Day of Arafat is: 'La ilaha illallahu wahdahu la sharika lah, lahul-mulku wa lahul-hamdu wa huwa 'ala kulli shay'in qadir.' The Prophet said this is the best supplication and the best of what he and all prophets before him have said. On Arafat, 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 Arafat is the greatest day of Hajj and the greatest day of the year for dua. The Prophet (peace be upon him) 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 supplication is the single most important act of Hajj.
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.'