Wellll, there's not a purported real world thing for either. They were pretty much not wine, mead, beer, or any other human food or beverage.

The kind of thing you're talking about is a fairly modern idea, and isn't exactly backed up by writings of the greeks. They had mead, and they had wines. They had beers. So why would they not directly mention them as such?

The word nectar probably stems from the roots of nek and tar, meaning to overcome death. Ambrosia has a similar etymology from words meaning immortal or undying.

The food of the gods was pretty well established to be something that humans didn't have access to. The myth of Ambrosia the nymph shows that it was never of mortal origin. And the Odyssey specifically compares wine to ambrosia and nectar, which again points to wine not being the same.

Both, however, were definitely liquids. They were drunk, and used to anoint, or even bathe in.

I don't think I've ever seen any translation of any myths of the greeks that indicates that nectar and/or ambrosia were something from the real world at all. Every mention of them distinctly depicts them as being divine and not simply a confused version of human drinks.

Now, if you want to ignore all of that, and guess at what the origins of the myths might have been built from, and you want to ignore the possibility that those myths were completely fabricated rather than being distorted stories of real people (which it's fairly likely that they aren't distorted history), then mead would be a good pick. But, so would entheogens like mushroom teas, or any of the consciousness altering plants extracted.

I would even hazard that, assuming we ignore the same things for this, that a more specific real world substance would be meads made from honeys that are tainted with hallucinogens. We know that "mad honey" was available to the greeks, and that the greeks made use of the kind of plants that bees would access to make "mad honey" in the first place.

The use of entheogens (hallucinatory or not) to connect with or become divine isn't exactly a rare thing. That the greeks may have simply taken it as granted that the gods would have some kind of "magic" food or drink is more likely than them having a distorted history passed down via oral writ.