What is the most important Commandment?

The first commandment in all the monotheistic religions (Judaism, Christianity and Islam) is one and the same. From the ten commandments (Exodus 20:1-17 and Deuteronomy 5:6-21), we see the decree:
Thou shall not take any god except one God.
Thou shall make no image of God.

In Deuteronomy chapter 6, verses 4-5, the first commandment is emphasized, and this has become known as Shema Yisrael or just Shema based on the first two words of the passage.

Hear, O Israel! The Lord our God is One God!
Therefore you shall adore the Lord your God 
with all your heart,
with all your soul, 
with all your mind,
and with all your strength.
[ Deuteronomy 6:4-5, Mark 12:29-30, Quran 3:18 ]