The best of supplications is the supplication on the day of ‘Arafah and the best which I and the Prophets before me said (is) :
لَا إِلَهَ إِلَّا اللهُ ، وَحْدَهُ لَا شَرِيكَ لَهُ ، لَهُ الْمُلْكُ وَلَهُ الْحَمْدُ ، وهُوَ عَلَى كُلِّ شَيْءٍ قَدِيرٌ
Laa ilaaha ill-allaahu, waḥdahu laa shareeka lah, lahul-mulku wa lahul-ḥamdu, wa huwa ‛alaa kulli shay’in qadeer.
None has the right to be worshipped except Allah, alone, without partner. To Him belongs sovereignty and all praise and He is over all things omnipotent.
Note: Whoever recites the above dua’ a hundred times during the day, has the reward of freeing ten slaves, a hundred good deeds are recorded for him and a hundred bad deeds are wiped away and he had gained refuge from the devil that day until evening and nine shall come with anything better except someone who has done more.