Is it obligatory for a Muslim to follow one of the four schools of thought, and if so, which is the best among them, and is it true that the school of Abu Hanifa is the most widespread?

A Muslim is not obliged to follow a specific one of the four schools of thought. The mujtahid (independent jurist) who is capable of deriving rulings from the Qur'an and Sunnah takes from them, and it is not permissible for him to merely imitate regarding what he believes to be true but contrary to such imitation. As for one who does not possess the ability to derive rulings, it is permissible for him to imitate someone whom he trusts, and he should ask until he achieves reassurance. Do not follow the sayings of the imams in all circumstances and times, for they may err. Rather, follow the truth from their sayings for which there is evidence.