The best supplication is the supplication on the day of 'Arafath                                                       

السلام عليكم ورحمة الله وبركاتة

بِسْمِ ٱللَّٰهِ ٱلرَّحْمَٰنِٱلرَّحِيمِ

The Prophet (SAW) said: "The best supplication is the supplication on the day of 'Arafat, and the best of what I and the prophets before me have said is: 'There is no god but Allah alone, without partner; to Him belongs dominion, to Him belongs praise, and He is powerful over all things'" (La ilaha illa Allah, wahdahu la sharika lah, lahul-mulk wa lahul-hamd, wa huwa 'ala kulli shay'in qadir).

الحمدلله