Who has the most right to lead the prayer (Imamah) and what is the evidence for this from the Quran and Sunnah? And is it permissible to say "illa Allah" (except Allah) during dhikr, and what does this dhikr mean?

The person most entitled to lead the prayer (Imamah) is the one knowledgeable of the rulings of prayer and a memorizer of the Book of Allah, based on the Prophet’s (peace be upon him) saying: "The people should be led in prayer by the one among them who is the best reciter of the Book of Allah. If they are equal in recitation, then by the one who is most knowledgeable of the Sunnah." The term "best reciter" (aqra'uhum) here does not refer to the one with the most beautiful recitation, but rather the one who has memorized the most. The Imam must be knowledgeable of the rulings of prayer because unexpected situations may arise that require correct action. The leadership of an ignorant person is not valid except for those who are equally ignorant, and when there is no one else suitable to lead. As for the phrase "illa Allah" (except Allah), it has not been mentioned as a standalone dhikr (remembrance) in the Shariah.