In The Court Of The Crimson King - King Crimson
I love Boston's debut album, and many of the other albums mentioned here, but ITCOTCK is number 1. Tell me, how many other debut albums mentioned here managed to define a music genre that is still running strong today? I doubt there's any that have had such an impact on music. But that's what King Crimson did when they released this album in 1969. In The Court of the Crimson King managed to define the genre now known as prog rock that was beginning to be formed, shaping it's growth in the years to come. Without this album, bands like Pink Floyd (compare Piper at the Gates of Dawn and A Saucerful of Secrets to what came after), Rush, Genesis, and Supertramp probably wouldn't have been the same as they ended up being. That's how much of a splash this album made.
The album is also really good, cutting out the blues elements that had defined rock in the 50s and 60s and replacing them with elements from jazz and classical music. Every song is great, with the weakest (but still interesting) section being the improv sections of Moonchild, and with the highs being found everywhere else. From the distorted vocals and saxophone of 21st Century Schizoid Man, to the soft lament of I Talk to the Wind, to the dark and funerary Epitaph, to the light lullaby of Moonchild, and finishing it off with the title track, which busts out the Mellotron. The moment when the each verse ends and Greg Lake belts out "In the Court of the Crimson KIIIIIIIIIIIIIIIIIIIIIIIIIIIIIINNNNNNNNNGGGGGGGGG" sends chills down my spine even today. It's fantastic, and will always be #1.