One of the questions that has been the subject of much discussion between Shi'i and Sunni scholars is the Imamate of the Most Excellent. The Sunni position is that if someone can be found to exist in the ranks of the ummah who is unequalled with respect to virtue, knowledge, and piety, someone less excellent than he may still legitimately become leader of the community and exercise the functions of successor to the Prophet, peace and blessings be upon him and his family.
In order to prove their point, they cite the caliphate of Abu Bakr and 'Umar, and they maintain that although 'Ali b. Abi Talib, peace be upon him, was present at the time and his worthiness and perfection were far more apparent than those of anyone else, the Companions nonetheless selected Abu Bakr as successor to the Prophet.
The Shi'ah believe that the Imamate constitutes an extension of prophethood in its spiritual dimension. The one who after the death of the Prophet is to serve as an authority for the Muslims in their learning the ordinances and principles of religion, who is to settle newly occurring problems for which no precedent can be found in the Qur'an and the Sunnah, whose words are to be a decisive criterion such a one must indubitably be more excellent than all others in his virtues and perfections. When God selects someone as the teacher of humanity and the guide of the ummah, to expound His laws, to interpret the complexities of the Qur'an, and to defend the truth and develop the personality of the ummah, He entrusts this position to an exceptional and inerrant person who is utterly unique in his spiritual qualities, his outer and inner attributes, his communication with the world of the unseen. Such a person perceives the inner truth of things with his inner eye and is always oriented to the truth in such a way that his faith is never corrupted and his deeds never deviate from the right path.
The Imam is therefore the most excellent being of his time, the foremost of all his contemporaries. Imam al-Rida, peace be upon him, says the following concerning the distinctive qualities of the Imam:
"The Imam is utterly free of sin and pure of all fault. He is celebrated for his knowledge and his forebearance. His existence is a source of pride to the Muslims, of anger to the hypocrites, of perdition to the unbelievers. The Imam is unique in his age, in the sense that no one can attain his rank. No scholar can come within range of his knowledge, and he is unequalled in all his qualities. He possesses all virtues and worthy attributes without any striving on his part, and he is adorned with all lofty characteristics. This is a great gift bestowed on him by God in His generosity." 
 al-Kulayni, al-Kafi, Vol. I, p.200.