Who are the best of this nation after the Messenger of Allah, may Allah bless him and grant him peace, and is it permissible to favor some Companions over others?

The best of this Ummah after the Messenger of Allah, peace and blessings be upon him, is Abu Bakr al-Siddiq, may Allah be pleased with him. Allah Almighty singled out all the Companions with the companionship of the Messenger of Allah, peace and blessings be upon him, and with working to support His religion. He described them with exalted qualities that indicate their elevated status, as He said: "Muhammad is the Messenger of Allah; and those with him are strong against disbelievers, merciful among themselves. You see them bowing, prostrating, seeking bounty from Allah and [His] pleasure. Their mark is on their faces from the trace of prostration. That is their description in the Torah. And their description in the Gospel is as a plant which produces its offshoots and strengthens them so they become firm and stand upon their stalks, delighting the sowers - so that He may enrage by them the disbelievers. Allah has promised those who believe and do righteous deeds among them forgiveness and a great reward." [Al-Fath:29].

The Ahadith have indicated the preference of some Companions over others. The best of them are the four Rightly Guided Caliphs, then the remaining ten promised Paradise, then those who witnessed Badr, for the Prophet, peace and blessings be upon him, said concerning them: "Do what you wish, for you have been forgiven."