The day of Arafah, which is the ninth day of dhul-hijjah. That’s one of the 10 days that some of the scholars said was the best day out of these 10 days because they have arafah.
The best dua that people made the prophets before Rasul’allah including the Rasulullah is that they used to say:
لاَ إِلَهَ إِلاَّ اللَّهُ وَحْدَهُ لاَ شَرِيكَ لَهُ لَهُ الْمُلْكُ وَلَهُ الْحَمْدُ وَهُوَ عَلَى كُلِّ شَيْءٍ قَدِيرٌ
Lā ilāha illallāh, waḥdahu lā sharīka lahu, lahul-mulku wa lahul-ḥamdu, wa huwa ‘alā kulli shai’in qadīr.
None has the right to be worshipped but Allah, alone without partner. To Him belongs all that exists, and to Him is the praise, and He is powerful over all things