Is it permissible to insult Ali ibn Abi Talib, and who has a higher rank: Omar or Othman?

It is not permissible to insult Ali, may God be pleased with him, or any other Companion. Insulting Ali is an act of disbelief if it impugns his religion and uprightness, due to the texts that indicate his virtue and status. Among his merits is that he was the first of the young boys to embrace Islam, the cousin of the Prophet, peace and blessings be upon him, the husband of his daughter Fatima, and the father of Al-Hasan and Al-Husayn, the two fragrant flowers of the Messenger of God. He must be loved and befriended without exaggeration or neglect.

As for the preference among the Companions, the Sunnis agree on preferring Abu Bakr, then Umar, may God be pleased with them both. However, preferring Uthman over Ali, may God be pleased with them both, is not a matter of consensus; rather, it is the view of the majority of Sunnis, while some of the early generations preferred Ali.