Who was the first Abbassid Caliph?

The Abbassids were ruled by Kings who styled themselves Caliphs (since all previous rulers of the Moslem Empires had done similarly. The First Abbassid Caliph is Abu Al-Abbas As-Saffah. He ruled from 750 CE to 754 CE and was succeeded by the Abbassid Al-Mansur (there is a different Umayyad Al-Mansur in Spain in the 900s).