Who was the oldest being ever to have existed in Greek mythology? In other words, were there a set of beings before all of the primordial gods? Who was to have created the primordial gods? What was the beginning of everything? If this is a chicken and the egg paradox, that could be an answer.
*By primordial gods I mean Gaea, Ouranos, and Pontus.
Hesiod's Theogony is widely regarded as the most ubiquitous of the Greek "creation myths".1 One of the earlier passages reads
(ll. 116-138) Verily at the first Chaos came to be, but next wide-bosomed Earth, the ever-sure foundations of all 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; but of Night were born Aether and Day, whom she conceived and bare from union in love with Erebus.
Chaos is sometimes depicted as an entity rather than a deity of some sort - the thing that existed before the universe. If you ascribe personification to it, then it counts as the first being. However, if not, then the so-called primordial gods were first:
Aether and Day are also considered by most authors to be two other of the primordial gods, though not created directly from Chaos.
1 There are variations on the idea. For example, Gaius Hyginus, a Roman scholar of Greek mythology, wrote that Chaos was formed from Mist, treating Chaos more as a being and Mist as something similar to Hesiod's depiction of Chaos. Hyginus is also notable for treating Earth as the child of Day and Aether, making the four deities born of Chaos and Caligine (a related but not well described entity) be Night, Day, Erebus, and Aether.
This is just to supplement @HDE 226868 's excellent answer:
Aristotle, quoting Hesiod, notes the implication that Love (attraction) must be the prime force of creation:
And Hesiod says, “ First of all things was Chaos made, and then/Broad-bosomed Earth . . ./And Love, the foremost of immortal beings,” thus implying that there must be in the world some cause to move things and combine them.
"Hesiod... assumed Love or Desire as a first principle in things."
Aristotle uses literally calls love (erota) and desire (epithumian) the origin (arkein) of all things (ouseen).
However, this is a philosophical conception of myth, and may be related to the Taoist idea that "out of two came all the myriad things", which is also a description of the symbolic system of the I-Ching.
The implication is that there must be two primordial things, such as Gaia and Tartarus, as HDE notes, although the type of generative interaction (combinatorial) may be more easily conceptualized in the subsequent unions, and offspring, of Gaia and Ouranos.
But what about Mist (Caligine) mentioned by Hyginas? While this may or may not be a deity/being, it certainly seems somewhat personified and Hyginas claimed that chaos came out of Mist... So it would seem that the earliest theorized entity/state of existence was some kind of Mist.
It depends who you ask.
As others have said, according to Hesiod the oldest one that existed was Chaos, and then Earth came to be.
According to Orpheus though, originally there was an Egg out of which the Universe came, but before the Universe, Eros (Love) came out of it, making Love the oldest one.
Finally, Plato says another version as he talks about a benevolent, omnipotent, Creator, that created both Heaven, the Universe and the 'gods'.
Chaos the void, then came Tartarus the abyss, Gaia the earth, Eros, Erebus the darkness, and Nyx, night. I'm assuming that was the parts of the world after that there is 7 actual gods and goddesses that came one being Aphrodite, she was there before Chronos and Zeus.
