The best of supplications is the supplication on the day of ʿArafah 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
Lā ilāha illā llāhu waḥdahu lā sharīka lahu, lahu l-mulku walahu l-ḥamdu wahuwa ʿalā kulli shayʾin qadīr
None has the right to be worshipped except Allah, alone, without partner. To Him belongs all praise and sovereignty and He is over all things omnipotent.