Who is the Greatest

The Holy Spirit is greater than the devil, by an immense margin.

The Devil is a fallen angel. He can only intervene on earth in one place at a time. This is severely limiting. The devil has an army of fallen angels, but it is not a united force. They are proud and independent and do not always do his will. The devil is hard pushed to get his work done on earth.

The Holy Spirit is divine. He was powerful enough to create the entire universe in just a moment of time. He is not limited to one place. He can be active throughout the entire earth, because he is not limited by space and time. The Holy Spirit can do greater things than Jesus did, and that it is a hard act to follow (John 14:12). He can convict the world of sin (John 16:8).

A struggle between the Holy Spirit and the devil there is not contest. The enemy is totally outclassed.