The Prophet (ﷺ) said: The best supplication is the supplication on the day of 'Arafah. The best dua that the Prophet (ﷺ) recited is -

لَا إِلَهَ إِلَّا اللَّهُ وَحْدَهُ لَا شَرِيكَ لَهُ، لَهُ الْمُلْكُ وَلَهُ الْحَمْدُ وَهُوَ عَلَى كُلِّ شَيءٍ قَدِيرٌ

Laa ilaaha ill-Allahu, wahdahu laa shareeka lahu, lahul-mulku wa lahul-hamdu, wa huwa ‛alaa kulli shay’in qadeer

None has the right to be worshiped except Allah, alone, without partner. To Him belongs sovereignty and all praise and He is over all things omnipotent.

Reference: Hasan (Albani). Tirmidhi: 3585