Huge fan of all bands, but “Let It Bleed” is mostly post-Brian Jones, and apart from a couple great songs, is a very boring album.
And “Dark Side of the Moon” is post-Syd Barrett, and while it’s an album everyone should hear at least once, it lacks the improvisation and experimental qualities of Pink Floyd’s earlier work, as well as has a very simplified rhythm section which lacks the energy of their earlier work.
So my answer is a tie between The Doors debut and the Led Zeppelin debut.
If you want to hear The Stones at their best, get “Aftermath” (the Stones’ first album with no cover versions), and “Their Satanic Majesties Request” (their psychedelic album). Both albums feature a lot of input and instrumentation from their founder Brian Jones.
If you want to hear Pink Floyd at their best, get “The Piper at the Gates of Dawn” (their debut with original guitarist and songwriter Syd Barrett, best heard as the mono mix), and “Meddle” (the album with the song “Echoes”, their best song without Syd).