Some of the very best stereo recordings were done in the late 1950's. It is a bit sad that the state of the art has not advanced that much. For example, a number of 6-eye Columbia's from that time are incredible sounding, such as Ellington's "Blues in Orbit" (1959); Brubeck's "Time Out" and Goodman's "Benny in Brussels (1958). Another beautifully recorded stereo recording from that era is Armstrong's "Louis Armstrong Plays King Oliver." I use these as demonstration records.
Some standard audio demonstration recordings from the 1960's include "Belafonte at Carnegie Hall" and the Weaver's "Reunion at Carnegie Hall 1963."
There are plenty of mono records that sound great, but, being mono, they give away their age. Still, who is not amazed by Rollins' "Saxophone Colossus" ?
For classical, I like the Cozart/Fine recordings for Mercury.