The Piper at the Gates of Dawn is the answer.

Pink Floyd’s debut album is The Piper at the Gates of Dawn, released in August 1967 in the UK (October in the US, but the UK date is the canonical one).

It’s the only full album with Syd Barrett as the main songwriter, and it sounds nothing like the Floyd most people know. Think psychedelic whimsy, spacey pop, and a lot of weird sound effects. “Astronomy Domine” and “Interstellar Overdrive” are the standouts. Not for everyone, but essential if you want to understand where they started before they became the Dark Side of the Moon band.