Who is the oldest out of the big three gods?

In Greek mythology, the oldest among the "big three" gods—Zeus, Poseidon, and Hades—is Hades. He is the eldest brother, born to Cronus and Rhea before Zeus and Poseidon. While Zeus is often seen as the primary deity ruling the sky and the heavens, Hades presides over the underworld and is associated with the afterlife.