Here’s some Socratic reasoning: Tomorrow Never Knows by The Beatles is an R&B tune because it has a hypnotic, driving beat. It is also a folk song because it’s carried by simple strumming on an acoustic guitar. But it’s pretty heavy, so you could argue it’s a rock song. Or compromise and call it folk rock.
Then again it’s very trippy, so really it’s psychedelic rock – or maybe even early prog rock, since it lifts from other genres and quotes strange modalities.
Or you could say it’s all of those things, which happens to nicely sum up The Beatles. Now, if you can say a single song can represent the diverse canon of the best group of all time, then obviously that song is the best song of all time.