The Best Supplication

The Messenger of Allah (ﷺ) said: 

لَا إِلَهَ إِلَّا اللَّهُ وَحْدَهُ لَا شَرِيكَ لَهُ لَهُ الْمُلْكُ وَلَهُ الْحَمْدُ وَهُوَ عَلَى كُلِّ شَيْءٍ قَدِيرٍ

"The best supplication offered is on day of 'Arafah. And the best that I and the Prophets before me uttered is Laa Ilaaha Ilal-llaah, Wahdahu Laa Shareeka Lahu, Lahul-Mulk Wa Lahul-Hamd Wa Huwa 'Alaa Kulli Shay'in Qadeer."

[Jaami'ut-Tirmithee no 3585, Al-Albaanee graded it as sound inSahihul-Jaami' no 3274 and Sahihut-Targheeb no 1536]