Music for string orchestra or large string ensemble

[You did it]

Ruggles - Portals
Xenakis - Shaar, Aroura
Guerrero - Oleada
Bartok - Divertimento
Penderecki - Threnody (probably the most famous high-level dissonant work of all time?  ), Polymorphia
Harrison - Suite For Symphonic Strings

[Mirror Image]

A magical piece, I have the recording from Vernon Handley and the Royal Philharmonic Orchestra

You mean the only recording of the Celtic Symphony.

[Cato]

Many great recommendations so far, but no one has yet mentioned one of the most disturbing, maniacal, and soul-stabbing        scores for strings ever composed:

Bernard Herrmann's...

https://www.youtube.com/v/is2sgWRK7D0

[TheGSMoeller]

Strauss: Metamorphosen - I know the OP mentioned it, but it's one of the best!
Janacek: Suite for String Orchestra
Janacek: Idyll - the 5th movement Adagio alone is worthy of being listed in the great string works.
Glass: Symphony No. 3
Respighi: Ancient Airs and Dances No. 3

[springrite]

Diamond: Rounds for String Orchestra

Barber: Serenade For Strings, Opus One

[Mirror Image]

Diamond: Rounds for String Orchestra

YES! A great work, Paul.

[XB-70 Valkyrie]

Thanks for the many interesting suggestions. Now all I need is a mega box set with all of them! I will certainly follow up on these suggestions. 

[Biffo]

You mean the only recording of the Celtic Symphony.

I am sure you are right but if I had said that someone would have dredged up a recording by the Novosibirsk Philharmonic.

Edit: The work has been recorded before, by Walter Collins and his London Promenade Orchestra. It is on a  Paxton 10" mono LP (GTR121) and is a bit of a rarity.

[pjme]

Two longtime favorites:

https://www.youtube.com/v/n91SwxVlNKA

https://www.youtube.com/v/GZB7dCLJ6cM

And -more or less - in the same vein:

https://www.youtube.com/v/vLPQfWGwK5I

https://www.youtube.com/v/TAJrTVo0Zdw

P.