Yamenko I love you, but Close to the Edge isn't Yes' masterpiece.
South Side of the Sky takes the cake for them. And I'd say all of Thick
as a Brick could be considered one song really, it's hard to judge the
first and second parts...but if you do need to choose I agree with you
about the first one.

I'd also say that Floyd's masterpiece is Echoes.

Procol Harum - In Held 'Twas in I
The single greatest progressive rock track ever created. When it's
performed with the Edmonton Symphony Orchestra it is the single most
amazing song in the history of mankind. ;]

Emerson Lake and Palmer - Tarkus

Sergei Rachmaninoff - Piano Concerto no. 2 in Cm (especially Part I)

Yann Tiersen - La Chute

Therion - The Siren of the Woods

Radiohead's most epic song WOULD be We Suck Young Blood if they
had jammed out on the climactic part like 4 minutes into it. However,
the world is not a perfect place and because they don't it makes it one
of the worst songs ever made, simply because it gives me the worst
case of blueballs every time I listen to it. Goddamnit I'm pissed just
thinking about it.

.opt

PS, actually now that you mention I agree with Street Spirit for
Radiohead.