If we are talking about core beliefs and not about the Five Pillars, the five most important beliefs of devout Muslims are:
Complete monotheism. It is very important to Muslims that they should not put anything or anyone on the same level as God. As an example of this, they reject the Christian idea of the Trinity.
Belief in the existence and the importance of angels. God uses them to do his will.
Belief in the prophets. Islam believes in many prophets, going back to Abraham. Of all of these, Muhammad was the last and the only one whose message did not get corrupted.
Belief in the scriptures. Muslims believe they are the word of God given to Muhammad.
Belief in the final judgment. On this day, God will hold all people responsible for their actions.