The most important songs in Prog rock history

The Who - A Quick one, While He's Away
A nine minute PROGRESSIVE rock-opera with different sections and amazing vocal melodies. I'm pretty sure that without this song there wouldn't be any songs like Close to the Edge, Supper's Ready or Thick as a Brick. This is actually the first "prog epic". ... or proto-prog at least.

Frank Zappa & The Mothers of Invention - Return Of The Son of the Monster Magnet
Let's face it, NOBODY had ever done anything like this before Zappa. The album "Freak out!" even inspired The Beatles to write more experimental songs.

The Beatles - Tomorrow Never Knows /A Day In The Life/Strawberry Fields Forever
These three songs are not only extremely experimental, but I consider A Day in The Life also very progressive. A truly innovative masterpiece.

I'm pretty sure that these are the most important songs in the prog rock history. Without these songs, there probably wouldn't be any progressive rock.

One more song I will add is Pink Floyd Interstellar Overdrive