Zeus' biggest rival is Poseidon. In Greek mythology, Zeus and Poseidon are both powerful gods, but they often clash due to their different domains. Zeus is the god of the sky and thunder, while Poseidon is the god of the sea. Their rivalry is depicted in various myths and stories, where they compete for power and influence. Poseidon's control over the oceans and his ability to create storms make him a formidable rival to Zeus, leading to conflicts and tensions between the two gods.