http://www.lankarani...aq/ideology.php
Q2: Is it of the essentials of faith to believe that Imam Ali (a.s.) is superior to all prophets?
A2: No, it is not of the essentials of faith.
Q3: What are the different degrees of prophethood? Imam Ali

is not higher in rank than Prophet Muhammad (saws), but he is higher than the other prophets. What is the basis of this claim?
A3: Prophets can be divided into two categories: 1) The Arch-Prophets who were sent on a mission by Allah (swt). They are considered as the prophets of Law and religion. They include Noah
, Abraham
, Moses
, Jesus
and Muhammad (saws), whose religions were spread in a wide area at their times. The Last Prophet, Muhammad (saws), is the last of the divine prophets who brought the universal religion, which will remain until the Last Day. There is no prophet after him. 2) The second category includes those prophets who propagated the religions of the Arch-Prophets. The extent of their prophethood and mission was not so great. Imam Ali
is higher in rank than other prophets, because of his Imamate, but he is not higher in rank than the Prophet Muhammad (saws), because Muhammad (saws) was both Prophet and Imam and he was the best of all creations.
Ignoring the issue that Ibrahim

was also an Imam and a Prophet, it's clear that this view is not in fact an essential part of shia beliefs. If it were as simple as using the Quran to prove that, then it would be an essential part precisely because of that fact.