The youngest son of Cronus and Rhea who overthrew his father was Zeus.
In Hesiod’s Theogony, Cronus learns from Gaia and Uranus that one of his own children will overthrow him, just as he had overthrown Uranus. To prevent the prophecy, he swallows Hestia, Demeter, Hera, Hades, and Poseidon at birth. When Zeus is born, Rhea hides him in Crete and gives Cronus a stone wrapped in swaddling clothes instead.
Zeus grows up in secret, then forces Cronus to disgorge his siblings. He frees the Cyclopes and the Hundred-Handers, who help him in the Titanomachy, the war between the Olympian gods and the Titans. Zeus and his allies defeat Cronus and establish a new divine order, with Zeus as ruler of the sky and king of the gods.
A common mix-up is confusing Zeus with Cronus: Cronus was the father who overthrew Uranus, while Zeus was the youngest child who later overthrew Cronus. Hades and Poseidon were Zeus’s older brothers, not his father’s conquerors.