Do You Have to Follow a Madhhab?
Question

 

Is it mandatory for a Muslim to follow a specific madhhab (Maliki, Hanafi, Hanbali, etc.)?
If it is so, what madhhab is the best? Is it true that Abu Hanifa's madhab is the most followed in the Muslim world?
Praise be to Allah.Is it obligatory to follow a madhhab?
It is not obligatory for a Muslim to follow any particular madhhab among the four madhhabs. People vary in their level of understanding and ability to derive rulings from the evidence. There are some for whom it is permissible to follow (taqlid ), and indeed it may be obligatory in their case. There are others who can only follow the Shar`i evidence. 
In Fatawa al-Lajnah al-Daimah, this question was answered in a detailed manner, which is worth quoting here in full. 
Question: 
What is the ruling on following one of the four madhhabs in all cases and situations? 
The Committee replied: 
“Firstly: the four madhhabs are named after the four imams – Imam Abu Hanifah, Imam Malik, Imam al-Shafi'i and Imam Ahmad. 
Secondly: These imams learned fiqh (jurisprudence) from the Quran and Sunnah, and they are mujtahidin in this regard. The mujtahid either gets it right, in which case he will have two rewards, the reward for his ijtihad and the reward for getting it right, or he will get it wrong, in which case he will be rewarded for his ijtihad and will be forgiven for his mistake. 
Thirdly: the one who is able to derive rulings from the Quran and Sunnah should take from them like those who came before him; it is not right for him to follow blindly (taqlid) when he believes that the truth lies elsewhere. Rather he should follow that which he believes is the truth. It is permissible for him to follow in matters in which he is unable to come to a conclusion based on the Quran and Sunnah and he needs guidelines concerning a particular issue. 
Fourthly:  Whoever does not have the ability to derive rulings himself is permitted to follow one whom he feels comfortable following. If he is not comfortable following him then he should ask until he finds someone with whom he is comfortable.  
Fifthly:  From the above it is clear that we should not follow their opinions in all situations and at all times, because they may make mistakes, but we may follow their views that are sound and are based on the evidence.” (Fatawa al-Lajnah, 5/28 )
It says in Fatawa al-Lajnah, no. 3323: 
“Whoever is qualified to derive rulings from the Quran and Sunnah, and has strong knowledge in that regard, even if that is with the help of the legacy of fiqh that we have inherited from earlier scholars of Islam, has the right to do that, so he can act upon it himself and explain it in disputes and issue fatwas to those who consult him. Whoever is not qualified to do that has to ask trustworthy people so that he may learn the rulings from their books and act upon that, without limiting his asking or his reading to one of the scholars of the four madhhabs. Rather people refer to the four imams because they are so well known and their books are well written and widely available. 
Whoever says that it is obligatory for the learned people to follow the scholars blindly in all cases is making a mistake and being inflexible, and is thinking that these learned people are inadequate, and he is restricting something that is broad in scope. 
Whoever says that we should limit following to the four madhhabs is also mistaken, because he is restricting something that is broad in scope with no evidence for doing so. With regard to the common (i.e., uneducated) man there is no difference between the four imams and others such as al-Layth ibn Sa’d, al-Awza’i and other fuqaha.” (Fatawa al-Lajnah, 5/41) 
It says in Fatwa no. 1591: 
“None of them called people to follow his madhhab, or was partisan in following it, or obliged anyone else to act in accordance with it or with a specific madhhab. Rather they used to call people to follow the Quran and Sunnah, and they would comment on the texts of Islam, and explain its basic principles and discuss minor issues according to general guidelines, and issue fatwas concerning what people asked about, without obliging any of their students or anyone else to follow their views. Rather they criticized those who did that and said that their opinions should be cast aside if they went against a sahih hadith. One of them said: “If the hadith is sahih then that is my madhhab.” May Allah have mercy on them all. 
It is not obligatory for anyone to follow a particular madhhab, rather we should strive to learn the truth if possible, or to seek the help of Allah in doing so, then to rely on the legacy that the earlier Muslim scholars left behind for those who came after them, thus making it easier for them to understand and apply the texts. Whoever cannot derive rulings from the texts etc for some reason that prevents him from doing so should ask trustworthy scholars for whatever rulings of shari’ah he needs, because Allah says (interpretation of the meaning): 
“So ask the people of the Reminder [Scriptures — the Tawrat (Torah), the Injil (Gospel)] if you do not know” [al-Anbiya 21:7]
So he has to strive to ask one whom he trusts among those who are well known for their knowledge, virtue, piety and righteousness.” (Fatawa al-Lajnah al-Daimah, 5/56 )
Which madhhab is the most popular?
The madhhab of Abu Hanifah (may Allah have mercy on him) is the most widespread madhhab among the Muslims, and perhaps one of the reasons for that is that the Ottoman caliphs followed this madhhab and they ruled the Muslim lands for more than six centuries. That does not mean that the madhhab of Abu Hanifah is the most sound madhhab or that every ijtihad in it is correct, rather like other madhhabs, it contains some things that are correct and some that are incorrect. What the believer must do is to follow the truth and what is correct, regardless of who says it.”
For more, please see this category: Schools of Thought and Sects
And Allah knows best.