In the Court of the Crimson King and In the Wake of Poseidon
So far, these are the only two I have, but I've noticed that they're eerily similar.
The first real track on ITWOP after the short intro, "Pictures of a City" sounds suspiciously like "21st Century Schizoid Man" (the 1st track off ITCOTCK, coincidentally) to me. The opening riff, the vocal section, the spazzy breakdown in the middle, the weird free jazz section, and how they both come back to the opening riff near the end.
"Cadence and Cascade" off ITWOP sounds like "I Talk to The Wind", slow, soft ballads with a lot of flute parts.
Also, the two title tracks are remarkably similar sounding. They both have the opening mellotron, followed by a slow, acoustic section with the vocals. Even the melody sounds alike.
The three "Peace" songs on ITWOP remind me a lot of "Moonchild". This could be for a number of reasons; the acoustic guitar, the vocal melodies, or maybe the sheer uncomfortableness.
The remaining songs, "Epitaph" off ITCOTCK, "Cat Food" and "The Devil's Triangle" off ITWOP don't really sound much alike.
I like both of these albums a lot, but they just struck me as being strangely similar. If you own them, I encourage you to dig both of them out and give them a listen back to back. Maybe it's just me.