Hard to pin point one, but I personally think it was quite some time prior to 'In The Court Of The Crimson King' myself. This was perhaps an early consolidation of themes found in early albums, such as;
John Coltrane- A Love Supreme (a very early concept album, with such depth, continuity and commitment it's hard for me to just plainly label it 'jazz')
Graham Bond Organisation- The Sound Of '65 ( an early attempt at jazz/ blues rock)
Joe Harriott/ John Mayer- Indo Jazz Fusions 1 and 2 (could be wrong about this, but this is certainly the first time I've seen the word 'fusion' crop up on an album- again, it's musically highly developed, and fusions of different styles was surely the whole purpose of prog, initially?)
The Yardbirds- Roger The Engineer (a pioneering album, and hugely varied, with psych pop, blues rock, gregorian chants, guitar instrumentals and even a jazz influence creeps in Jeff Beck's wild guitar work)
The Who- A Quick One, While He's Away (not a prog album as such, but features a very early attempt at a song suite in the title track)
Frank Zappa- Freak Out!/ Absolutely Free/ ...Money/Lumpy Gravy (took hugely uncommercial music and themes into mainstream rock/pop)
The Beatles- Rubber Soul, Revolver, Sgt Pepper (hugely revolutionary, as they feature a band experimenting with a myriad of genres and utilising new production methods)
Vanilla Fudge- s/t (hugely influential, as it saw a band taking pop songs and creating something extremely new and challenging with them)
The Zombies- Oddesey And Oracle (arguably took the 'pop' three minute song idea to its maximum, with huge intricacy, brilliant themes, plus swathes of mellotron and pseudo classical themes)
Nirvana- The Story Of Simon Simopath/ The Pretty Things- SF Sorrow (both were hugely pioneering, as they had an ongoing story throughout a whole album)
Genesis- From Genesis To Revelation (more revolutionary than you'd imagine, with intricate song arrangements and a clear concept)
Touch- s/t (features some of the longest songs on an album up to that point, with much intricacy and musical experimentation)
I'm not going to pick just one album from these, yet I feel all of these (from 1964-68) feature traits that would soon be apparent in prog rock.
Edited by salmacis