Imam Ali is revered for his courage, honesty, unbending devotion to Islam, magnanimity, and equal treatment of all Muslims. For his admirers, he has thus become the archetype of uncorrupted Islam and pre-Islamic chivalry. Ahlus Sunnah regard him as the last of the rashidun caliphs, while the Rawafidh take him as the first caliph rejecting the Caliphate of Abu Bakr, Umar and Uthman, May Allah be pleased with them all. May Allah either guide, or curse the Rawafidh.
Virtues of Imam Ali عَلَيْهِ ٱلسَّلَامُ
The Prophet ﷺ said that no one but a believer would love Ali, and none but a hypocrite would nurse grudge against Ali.
Saheeh Al Muslim 78
This proves that whoever loves Imam Ali is a Mu'min with complete faith. While those who hate Imam Ali are none but Munafiqeen, they might even fall to Kufr for their enmity against Imam Haider e Karrar
Rasoolallah ﷺ said that "For whomsoever I am a Maula, Ali is his Maula. Oh Allah, love the one who loves Ali, and hate the one who hates Ali!"
Al Bidaya Wan Nihaya 18/668
This indicates the significance of Imam Ali in the Ummah of Rasoolallah ﷺ and this also indicates that indeed Imam Ali is the one loved by Allah and his Wali, same as Shaykhain Abu Bakr and Umar.
The Prophet (ﷺ) said to 'Ali, "Will you not be pleased from this that you are to me like Aaron was to Moses?"
Saheeh Al Bukharee 3706
This Hadith indicates that if there were any other Prophets after Rasoolallahﷺ, It would have been Imam Ali, this is because of his love and sacrifice for Allah and His messenger ﷺ
Someone had said that Imam Ali is not from Ahlul Bayt and it is not allowed to say Salam on him. Shaykh Al Islam Ibn Taymiyyah said: "Indeed Imam Ali is from Ahlul Bayt and there's no diference among the Muslimeen in it.
Rather he is the BEST OF AHLUL BAYT AND THE BEST OF BANI HASHIM AFTER RASOOLALLAHﷺ
Majmo'o Al Fatawa 3/496