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It's Zeus. In the actual ancient Greek Lore, Zeus has only been defeated ONE time, and it took 3 other Olympians allied against him to do it. It was Poseidon, Apollo, and one other of the 12 Olympians allied against him, and that's his only loss. On another occasion, two Olympians allied against him and he beat both of them simultaneously.
Chaos, Gaea, or Uranus could beat Zeus if they wanted to do so, but they mostly don't interact with the Olympians, and pretty much never interact with mortals.Chaos is literally Almighty, and is supposed to be "The Unknown God" who actually created all possible realities, so technically Chaos would win, but Chaos never interacts with lesser beings.
What do you think, Who teached poseidon how to swim?