What is the miracle of the Prophet Jonah (peace be upon him) by which all his people believed, given that the Quran is the greatest of the miracles of the Prophet Muhammad (peace and blessings be upon him)?

The criterion of merit is not measured by the number of believers who embrace faith through a person's efforts. Ibn Kathir mentioned that Yunus (peace be upon him) invited the people of Nineveh to Allah, but they refused. So, he departed in anger and warned them of punishment. When they realized the punishment was imminent, they went out to the desert with their children and livestock and implored Allah. Consequently, the punishment was lifted from them, as Allah Almighty states: ﴿فَلَوْلَا كَانَتْ قَرْيَةٌ آمَنَتْ فَنَفَعَهَا إِيمَانُهَا إِلَّا قَوْمَ يُونُسَ لَمَّا آمَنُوا كَشَفْنَا عَنْهُمْ عَذَابَ الْخِزْيِ فِي الْحَيَاةِ الدُّنْيَا وَمَتَّعْنَاهُمْ إِلَى حِينٍ﴾ [Yunus: 98] ("Was there any town that believed and profited by its faith, except the people of Jonah? When they believed, We removed from them the torment of disgrace in the life of the world and permitted them to enjoy for a while.") Our Prophet Muhammad (peace be upon him) will have the most followers on the Day of Resurrection, as stated in the Sunnah.