What is the most important reef builder?

Can you tell me, what is considered to be the most significant contributor to the formation and growth of coral reefs? Is it a specific type of coral, an organism that lives symbiotically within coral, or perhaps an environmental factor that plays a pivotal role in fostering the development of these vibrant underwater ecosystems? Understanding the key reef builder can help us appreciate the intricacies of coral reef ecosystems and the challenges they face.