The whole album is for Syd, but "Shine On You Crazy Diamond" is the explicit tribute.

“Shine On You Crazy Diamond” (Parts I–IX) is the one. It bookends the album, starts and ends with that iconic four-note synth lament, and the lyrics are unmistakably about Syd Barrett — the “crazy diamond” who burned out before the band took off. The title track “Wish You Were Here” is also about Syd in a more general, longing sense, but “Shine On” is the direct eulogy.

The story goes they were recording it in the studio, and a heavyset, bald man with shaved eyebrows walked in. Nobody recognized him. It was Syd. He listened for a while, said he could play some guitar, and then left. They didn’t see him again. That’s how close the tribute hits.

If you want the one song that says everything, start with “Shine On You Crazy Diamond.” The rest of the album fills in the feelings.