Who is the oldest Greek God according to mythology? Is it Poseidon or Aphrodite? I have read most of the Percy Jackson books but can someone point out the answer and the story that goes with it?
Well, if you consider Chaos as God, then Chaos. Otherwise, all together Gaia, Eros, Erebus and Nyx:
In Greek mythology, Chaos (Greek: Χάος), according to Hesiod, Chaos ("Chasm") was the first thing to exist: "at first Chaos came to be" (or was) "but next" (possibly out of Chaos) came Gaia, Tartarus, and Eros (elsewhere the son of Aphrodite). Unambiguously born "from Chaos" were Erebus (Darkness) and Nyx (Night).// Wikipedia
See also Hesiod's Theogony:
(ll. 116-138) Verily at the first Chaos came to be, but next wide-bosomed Earth [Gaia], the ever-sure foundations of all (4) the deathless ones who hold the peaks of snowy Olympus, and dim Tartarus in the depth of the wide-pathed Earth, and Eros (Love), fairest among the deathless gods, who unnerves the limbs and overcomes the mind and wise counsels of all gods and all men within them. From Chaos came forth Erebus and black Night[Nyx]; but of Night were born Aether (5) and Day, whom she conceived and bare from union in love with Erebus. And Earth first bare starry Heaven, equal to herself, to cover her on every side, and to be an ever-sure abiding-place for the blessed gods. And she brought forth long Hills, graceful haunts of the goddess-Nymphs who dwell amongst the glens of the hills. She bare also the fruitless deep with his raging swell, Pontus, without sweet union of love. But afterwards she lay with Heaven and bare deep-swirling Oceanus, Coeus and Crius and Hyperion and Iapetus, Theia and Rhea, Themis and Mnemosyne and gold-crowned Phoebe and lovely Tethys. After them was born Cronos the wily, youngest and most terrible of her children, and he hated his lusty sire.
Cronos was a bit afterwards.