Who is the supreme god in greak mythology?

There were different supreme Deities at different times in Greek Mythology

First was Chaos
then Erebus
then Aether
then Uranus
then Cronus
then Zeus

Zeus however shared the position with Posedion and Hades, although most accounts have him as the most deferred to.

A prophesy foretold that the child of Zeus and Metis would over throw Zeus as he had over thrown his father Cronus. Zeus got Metis pregnant and then swallowed her in an attempt to keep the prophesy from coming true. Unfortunately this didn't prevent the child Athena from being born. She broke free from Zeus' head fully formed and armoured. Fortunately she was female and therefore incapable of fulfilling the prophesy. (Apparently only men can rule in those days)

A similar tale is told of the nymph Thetis. Zeus and Poseidon had been rivals for her hand until Prometheus, warned Zeus of a prophecy that Thetis would bear a son greater than his father. For this reason, the two gods withdrew their pursuit, and had her wed Peleus. Her son was Achilles.