Very hard to pick a single one.

However, Bach's The Well-Tempered Clavier encompasses so many styles and so much skill it works as a paragon for most of the ideas expressed in modern day music, so I'll go with it.

Mate, don't be THAT guy - I know OP is not specific, and technically yes, all songs are fair play, so quite a bit of classical music would be on top, but I think it's quite obvious what he had in mind.

Guns n' Roses - Estranged

hard to say, one i really love is chandelier from SIA, her voice is amazing

So... you basically agress with me and my reasoning, but seems irked for some reason?

If I lied and mentioned a random single from a english band of the 60s and 70s, as it often happens, would it be more acceptable?

Omg! This is totally hard! Except at the same time .... not?

Objectively(that means i'm stating facts), it's Another Brick in the Wall. Pink Floyd was already the KING of metaphors with The Dark Side of the MOON, but he just took it a step further with A Wall. An other brick in the wall is pure musical geniusousity. Few artists can be such progressiveness yet so punk at the same time. And "We don't need no, education",the irony, because they said that wrong! wrong! it's like poetry to my ears. It says a lot about Pink Floyd that it is harder to discuss his best song than it is what song can be the best ever, because truely only his songs can fight on that mountain for supremacy.

https://www.youtube.com/watch?v=8YaA4o03y8k&ab_channel=ImmortalSengir

Not sure if the best but it is definitely up there for me.

Drake- The ride

Or

Kendrick Lamar - Sing about me/ dying of thirst

You wouldn't be really lying - as I said, although OP was not very specific, his own choice kinda set the tone - at least that's how I look at it, I'd be surprised if he really thinks that's actually the best piece of music of ALL time.