"The best of supplications is the supplication on the day of 'Arafah and the best which I and the Prophets before me have said is:
لاَ إِلَهَ إِلاَّ اللَّهُ وَحْدَهُ لاَ شَرِيكَ لَهُ، لَهُ الْمُلْكُ وَلَهُ الْحَمْدُ وَهُوَ عَلَى كُلِّ شَيْءٍ قَدِيرٌ
None has the right to be worshipped but Allah alone, Who has no partener. His is the dominion and His is the praise, and He is Able to do all things.
Laa 'ilaaha 'illallaahu wahdahu laa shareeka lahu, lahul-mulku wa lahul-hamdu wa Huwa 'alaa kulli shay'in Qadeer.
At-Tirmithi. Al-Albani graded it good in Sahih At-Tirmithi 3/184, and also Silsilatul-'Ahadith As-Sahihah 4/6.