Barber's Adagio for Strings

The Greatest version I have heard is the old Toscanini version. He was able wring all the emotion out of it while keeping the pacing. If mono bothers you Bernstein with NYPO is good choice. There is also the Schippers version. But the Toscanini is in class by itself. Hope this helps.

Get the Zinman/Baltimore. You'll be rewarded with a full CD of terrific Barber performances, not merely a good "Adagio" (there are many of those). Great sound.

If you like this piece, then look for the Barber arrangement for unaccompanied chorus (acappella) recorded by Robert Shaw. This music with voices in place of strings is one of my favorites.

It is titled "Agnus Dei", and is available on a Robert Shaw CD titled Evocation of the Spirit. The Tower Records link that includes samples to listen to is:
http://www.towerrecords.com/product.asp?pfid=1095833

If you did not know, this adagio is originally the second movment of a String Quartet by Barber. It is also fun to listen to in its original version. I have a recording by the Emerson string quartet.

I second what Sugarbrie said above. The Emerson is a fine rendition of it in its oirignal quartet form.
In addition to the Toscanini mentioned above, consider the St. Louis/Slatkin recording. When they toured Asia a few years back, the Adagio movement was such a hit that they got bugged to play it on every concert. Hence, they know it cold. It's also well recorded.