Answer: Poseidon
Some extra relevant information:
In Greek mythology, the elder brother of Zeus was Hades. Zeus, Hades, and Poseidon were three powerful gods known as the “Big Three” and sons of the Titans Cronus and Rhea. Hades, as the firstborn, was the ruler of the underworld and the realm of the dead. Zeus, in turn, became the ruler of Mount Olympus and the supreme god of the heavens. Poseidon, the second brother, was the god of the seas and waters. These three brothers divided the dominion over the world amongst themselves after defeating the Titans in a war known as the Titanomachy. Despite being the elder brother, Hades played a much lesser role in Greek mythology compared to Zeus, who often takes center stage due to his position as the king of the gods and numerous mythological tales associated with him.