Prog has probably been the most influential in all of my passionate relations with music..... it's funny I don't listen to much anymore.
So many have already mentioned these, but here goes:
Thick As A Brick
Relayer (though i'm a bigger fan of Sound Chaser and To Be Over, which has one of my all time favorite guitar solos.)
2112
Court of the Crimson King
Nursery Crime
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Um, I can't come up with any modern music for this list at all. I need to check out some of the stuff that is mentioned in this thread. Wait, is Radiohead actually considered prog? I don't really think so, but i'd vote for OK too. Mars Volta? I got into Francis the Mute for a bit, but totally lost interest after awhile and just haven't checked out their other work, though I may sometime. Tool? Ok, Tool is incredible, but I just can't dig into it enough emotionally to get attached, if you know what I mean, although lately i've been having a blast practicing drums to some Tool songs (not that I can play like him!).
Apparently I don't know anything new here. What happened to this music? It seems the digital age killed the kind of interaction necessary for bands to create sonic masterpieces like these bands once did...