The best of the Followers is Uways al-Qarani

It is narrated from Abd al-Rahman bin Abi Layla that on the day of Siffin a caller from among the companions of Sayyiduna Mu'awiyah (may Allah be pleased with him) called out to the companions of Sayyiduna Ali (may Allah be pleased with him): "Is Uways al-Qarani among you?" They replied: "Yes." So that man spurred on his mount until he joined the army of Sayyiduna Ali (may Allah be pleased with him). Then he said: I heard the Messenger of Allah (peace and blessings be upon him) say: "The best of the Tabi'in is Uways al-Qarani."

It is narrated from al-Asbagh bin Nubatah that he said: I was present with Sayyiduna Ali (may Allah be pleased with him) on the day of Siffin, and he was saying: "Who will pledge allegiance to me unto death - or he said: unto being killed?" Ninety-nine men pledged allegiance. Then he (may Allah be pleased with him) said: "Where is the one who completes (the number)? Where is the one I was promised?" The narrator says: Then a man came wearing worn-out woollen clothes, with his head shaved, and he pledged allegiance to him unto death and being killed. It was said: This is Uways al-Qarani. He kept fighting in front of him until he was martyred.
Imam al-Hakim says: On this subject there is an authentic narration from the Messenger of Allah (peace and blessings be upon him) through the Commander of the Faithful, Umar bin al-Khattab (may Allah be pleased with him).