The word 'ahsan' should probably be translated as 'most beautiful' rather than 'best'. The story of Yusuf is the most beautiful story because it demonstrates the power of forgiveness and patience even in the face of great wrong. Yusuf, as a prophet with a pure soul, persisted in patience to such a degree that he retained his purity when confronted by the treachery of his brothers, who sold him into slavery. He retained his pure character when he found himself in a pagan society, whose licentiousness caused his patron's wife to attempt to entrap him; an act for which he endured with monumental patience an unjust term in prison. Yusuf further retained his purity when he attained political power. As a political appointee of a kindly Pharaoh, he governed with justice and compassion. When his brothers came to him, he forgave them for he recognized that it was the shaitan who had really caused the strife between him and his brothers:
And he placed his parents on the dais and they fell down before him prostrate, and he said: O my father! This is the interpretation of my dream of old. My Lord hath made it true, and He hath shown me kindness, since He took me out of the prison and hath brought you from the desert after Satan had made strife between me and my brethren. Lo! my Lord is tender unto whom He will. He is the Knower, the Wise.
This story closely parallels the trials and triumphs of the Prophet Muhammad (pbuh) who also faced betrayal by his own family, i.e., his uncles and who endured banishment and war at the hands of the treacherous Meccans, who were his fellow clansmen. Like Yusuf, the Prophet endured these trials with enormous patience and forgave his clansman for he too realized that it was the Shaitan, who was trying to subvert the religion by disrupting the unity of the ummah. Forgiveness, persistent patience, compassion and a unified ummah are major characteristics of Islam, which caused the rapid spread of this religion. And, finally, Allah (swt), through the examples of His messengers (peace be upon them all) demonstrated to Adam, the vast dimensions of His own mercy to all of Bani Adam.
And Allah (swt) knows best!