Answered by Shaykh Amjad Rasheed
What do the scholars say about Mu`awiya? What do we say about one who deems him misguided or a disbeliever?

In the name of Allah, Most Merciful and Compassionate
The position of our Imams of the Ash`ari school of Sunni beliefs is that Sayyiduna Mu`awiya (Allah be pleased with him) is from the Companions of the Prophet (peace and blessings be upon him). All the Companions of high rank in the Shariah, because of the honor of having met our Noble Messenger of Allah (Allah bless him and give him peace), and the Prophet (peace and blessings be upon him) died pleased with them all. It is true that the ranks of the Companions differ in virtue, depending on who entered Islam earlier and who served the religion with their person and wealth more. [See: Taftazani, Sharh al-Aqa’id al-Nasafiyya and its commentaries; Laqani, Jawharat al-Tawhid, and its commentaries by Bajuri, Sawi, Tattan, and others]
The position of Sunni Islam is that the differences and disputes that took place between the Companions during their period of disagreement do not allow us to censure them nor to speak ill of them. This applies to both Sayyiduna Ali and Sayyiduna Mu`awiya (Allah be pleased with them both), and those who followed either of them. Even worse is to deem any of them to be misguided or (God forbid) a disbelieverexcept those whose apostasy was legally established.
What is entailed by having good opinion of the Noble Companions of the Messenger of Allah (Allah bless him and give him peace) is that if they sinned they hastened to repentance; and that through the blessing of their having been with the Messenger of Allah (Allah bless him and give him peace) they were honoured by death on Islam and in the best of states.
There is scholarly consensus, as is the position of Sunni Islam, that Sayyiduna Ali (Allah be pleased with him) was correct in the differences that existed between him and Sayyiduna Mu`awiya (Allah be pleased with him), and that these differences did not occur due to selfish desires or caprice but rather through each exercising their independent judgment (ijtihad). Sayyiduna Mu`awiya (Allah be pleased with him) erred, and thus has the reward of his ijtihad, and he was not sinful for his error.
This has been explicitly stated by the acknowledge imam of Sunni Islam in beliefs, Sayyiduna Abu al-Hasan al-Ash`ari (Allah be pleased with him), in his Maqalat, and was also the position of those after them. What some wrongfully attribute to Imam Ash`arithat he deemed Mu`awiya to be a sinner and misguidedis simply a misattribution [f: and, as Shaykh Amjad proved in a treatise he wrote on the subject, a case of deliberate textual corruption on their part].
Our duty is to be silent regarding the dispute between the Companions (Allah be pleased with them), and to have a good opinion of them all. In fact, Imam Ghazali (Allah be pleased with him) counted careless talk (khawd) about these matters to be from careless sinful talk (khawd fi al-batil) and sinful, as mentioned in his Ihya’.
As for some of the narrations in which some of them allegedly spoke ill of others, these are either very weak [f: or, in many cases, fabrications] or can be properly explained. Imam al-Haramayn al-Juwayni (Allah be pleased with him) [f: the teacher of Imam Ghazali] said regarding proper explanation of these differences, “And no one of religion can fail to do so.”
As for the one who declared Mu`awiya misguided or a disbeliever, such a person is an innovator (mubtadi`). Declaring him a disbeliever is much more dangerous: it could lead to falling into disbelief, for the Messenger of Allah (Allah bless him and give him peace) said, “If one of you declares their brother a disbeliever, then one of you is indeed a disbeliever.”
And Allah alone gives success.
Amjad Rasheed.
(Translated by Faraz Rabbani)