project

woah, that's a question impossible to answer ...

I'd say 21st century schizoid man by KC was way ahead of it's time and inspired tons of bands, it also was on one of the first prog records. (btw, the most important to prog, not the BEST)

The most important album is ITCOTCK - without question.

There are, of course, many other important albums, songs and musicians - I think you need to decide on an angle to take this from, or you'll end up with a long, sprawling, noodling mess.

A bit like a Yes album

Video:

I'd have to agree with Bebie and Cert here: although it is sort of silly to believe that there is any one song that is "most important" to progressive rock, "21st Century Schizoid Man" certainly comes close.  It is the opening track on the album that virtually "defined" the progressive rock movement.  And it contains almost all of the elements that "defined" (at least early on) "progressive rock": extended, thematic compositions, "symphonic" keyboards (particularly mellotron), non-standard and shifting time signatures, non-standard (for rock) instruments (including reeds, woodwinds, vibes), radical dynamic changes, radical sound textures, elements of rock, jazz, folk and experimental music, virtuoso musicianship, use of the recording studio and studio "tricks" as an important element in the creation of the overall sound, and esoteric and "fantastic" lyrics.

It really doesn't get much more "perfect" than that, and certainly not as early as 1969.

If I had to make other choices, my personal ones (and note everyone: my personal ones) would be The Musical Box and/or Return of the Giant Hogweed (Genesis), and any track on Close to the Edge (Yes).  These may not be as "important" to the genre as "21st Century Schizoid Man" in the "historic" sense, but, in my opinion, they equally "define" - in their own ways - what the "progressive" sound was in its early years.

Peace.