I am a long time fan of all their incarnations.
Their best are: Larks' Tongue in Aspic, Starless and Bible Black, and Red. These 3 are their most progressive and musically adventurous.
But all of their releases have something of interest.
King Crimson
The only knowledge I had of King Crimson was my first LP bought self titled Bad Company, and reading the bass player Boz came from Crimson. Fast forwarded 50 years and I am now hearing for the first time "in the court of the Crimson King". I know I wouldn't have liked what I love today, when I was 13. Amazing players and composers! Court is not dismissed, please remain seated.