Who Should Lead the Prayer?
Question

I would be very grateful if you answer my questions :
Q1: Could you tell me who has the most right to lead salah and could you back it up with the Quran and hadiths as well?
Q2: When we are doing ياikr is it ok to say (ill-Allah) and what does it mean?
Praise be to Allah.Who should lead the prayer?
The one who most deserves to lead the prayer is the one who has the most knowledge of the rulings on prayer and has memorized the most Quran. 
Abu Mas’عd al-Ansari said: The Messenger of Allah (peace and blessings of Allah be upon him) said: “The people should be led in prayer by the one who has most knowledge of the Book of Allah; if they are equal in knowledge of the Quran, then by the one who has most knowledge of the Sunnah.” (Narrated by Imam Muslim, 1530) 
What is meant by “the one who has most knowledge of the Quran” is not the one who can recite it most beautifully, but rather the one who has memorized more of the Book of Allah . This is indicated by the hadith of ‘Amr ibn Salamah who said: “I used to memorize those words – meaning the Quran – and it was as if it took root in my heart. When Makkah was conquered, then every tribe hastened to embrace Islam, and my father urged our tribe to hasten to embrace Islam. When my father returned (from the Prophet) to his tribe, he said, ‘By Allah, I have come to you from the true Prophet (peace and blessings of Allah be upon him)!’ The Prophet (peace and blessings of Allah be upon him) afterwards said to them, ‘Offer such-and-such a prayer at such-and-such a time, and when the time for the prayer becomes due, then one of you should pronounce the Adhan (for the prayer), and let the one amongst you who knows most Quran lead the prayer.’ So they looked for such a person and found none who knew more Quran than I, because of what I used to learn from the people. So they made me their imam (to lead the prayer) and at that time I was a boy of six or seven years.” (Narrated by al-Bukhari, 4051) 
We say that it is essential for the imam to know the rulings on prayer, because he may do something, such as not breaking his wudu, or missing out a rak`ah, and not know what to do about it, in which case he would make a mistake and cause the prayer of others to be imperfect or invalid altogether. 
Based on the hadith quoted above, some of the scholars said that the one who has more knowledge of fiqh should lead the prayers. 
Al-Nawawi said: 
“Malik, al-Shafi’i and their companions said that the one who has more knowledge of fiqh should take precedence over the one who knows more Quran, because what he needs of recitation is known but what he needs of fiqh may not be clear. There may arise during the prayer some matter where only a person who has complete knowledge of fiqh will be able to know the right thing to do. They said that this is the reason why the Prophet (peace and blessings of Allah be upon him) told Abu Bakr (may Allah be pleased with him) to lead the prayer rather than anyone else, even though he (peace and blessings of Allah be upon him) had stated that others had more knowledge of Quran than him. But some refuted this point by quoting the hadith that said that the Companions who had most knowledge of Quran were also those who had most knowledge of fiqh. But there is another hadith according to which the Prophet (peace and blessings of Allah be upon him) said: “If they are equal in knowledge of Quran, then the one who has most knowledge of the Sunnah,” which indicates that those who have most knowledge of Quran should be given precedence in all cases.” (Sharh Muslim, 5/177) 
Even though al-Nawawi differed with his imam al-Shafi`i with regard to the interpretation of the hadith, their words illustrate the basic principle that there was no one among the Companions who had a good knowledge of the Quran but was ignorant of the rulings of Shari`ah, as is the case among many people nowadays. 
Ibn Qudamah said: 
“If one of them has more knowledge of the rulings on prayer, and the other has more knowledge of matters other than prayer, then the one who has more knowledge about prayer should be given precedence.” (Al-Mughni, 2/19) 
The Standing Committee said: 
“If this is understood, then it is not permissible for one who is ignorant to lead the prayer unless he is leading others like him, if there is no one who is better qualified to lead the prayer.” (Fatawa Islamiyyah, 1/264) 
Is there a dhikr called Illa-Allah?
We do not understand what is meant in the question. The phrase “ill-Allah” is not dhikr on its own, and it is not mentioned in any dhikr on its own. Rather it comes as part of another phrase, namely La ilaha illa-Allah wahdahu la sharika lah, lahu’l-mulk wa lahu’l-hamd wa huwa ‘ala kulli shay’in qadir (There is no god but Allah Alone, with no partner or associate; His is the Sovereignty and to Him be praise, and He is able to do all things). And it appears in many other dhikrs.
For more, plase see these answers: 11465 , 60375 , 21564 , 101766 , 9933 , and 13465 . 
Please see also this category: Leading Prayer.
And Allah knows best.