This is a simple question to answer. God is great because he is selfless. He places him self below all, so that all may rise. Let me explain...
God is not some mystical fog floating through the universe. He is a actual person possessing great wisdom and power. He has power because he loves and accepts all things. He spoke and creation occured because all elements honor God because he speaks nothing ill. He still speaks and the universe expands as he dictates. God knows the end from the beginning. He knows where we will end up, but allows us still to live our lives without destroying our hopes.
God does not have us worship him out of vanity. His speech is so refined and our reception is so cluttered that he needs us to concentrate to hear his words--so he asks us to pray. With that focus he can send us words, feelings, promptings if we purify ourselves. He does not give us commandments because he is a usurper of power. He knows that following the basic commands that he gives will lead to freedom from the bondage of sin. Sin corrupts our mind or taints our thinking.
God is great because he promised us that we can have all that he does now have. That is a loaded promise. We may have all that he has and have all knowledge and power with him if we endure our life well. What is he promising us then? What does he own? Think about it.
God is great because he really is the father of our spirits and we are truly all brothers and sisters. What good dad could be anything other than great.