In the anime series "Full Metal Alchemist", what do Alchemists consider the greatest taboo?

In the anime series Fullmetal Alchemist, alchemists consider Human Transmutation to be the greatest taboo because it involves attempting to resurrect the dead or create life, actions that are seen as playing with the natural order of things and challenging the boundaries of human existence. Not only is this forbidden by alchemical laws, but it also carries severe consequences, often resulting in the creation of twisted, incomplete beings known as "homunculi." Furthermore, it is a practice that goes against the fundamental principles of equivalent exchange, as sacrificing a human life in an attempt to bring another back to life is considered deeply unethical.