Leading the prayers by one who does not do rukoo’ properly
Question

                If a brother who has better tajweed than the rest of the Congregation, however does not make Ruku (bowing) in prayer correctly,  can he be made or allowed to be imam?
Praise be to Allah.
    The one who is most entitled to lead the prayers is the one 
    who has most knowledge of the Book of Allaah and who has most knowledge of 
    the fiqh of prayer and is most keen to ensure that his prayer is correct. 
    That includes doing the essential parts and obligations of the prayer in the 
    required manner. But if he does not do the prayer or one of its essential 
    parts, such as rukoo’, properly, then the matter is subject to further 
    discussion: 

    -If he is unable to stand or bow 
    or prostrate for a valid reason such as sickness etc., then there is a 
    difference of opinion among the scholars. The most correct view is that his 
    leading the prayers is valid because of the general meaning of the hadeeth 
    of the Prophet  (peace and blessings of Allaah be upon him): “The people 
    should be led in prayer by the one who has most knowledge of the Book of 
    Allaah.” This is the madhhab of al-Shaafa’i (may Allaah have mercy on him), 
    as it says in al-Majmoo’, 4/264, and this was the view favoured by 
    Shaykh al-Islam Ibn Taymiyah (may Allaah have mercy on him) as was narrated 
    from him in al-Insaaf, 4/374.
If the reason is ignorance or stubbornness, 
    then his mistake should be pointed out to him. If he responds, all well and 
    good, otherwise it is not permissible for him to lead the people in prayer, 
    even if he is hafiz, because his prayer is invalid due to his failing to do 
    one of the essential parts of prayer. See Question no. 
    1875.