I guess it depends on whether you are looking for studio ’tricks’ for L R separation, things like Dark Side of the Moon are always highly thought of, as is Wish You Were Here.
Yellow as previously mentioned contains a lot of the same sort of studio pans, etc.
But if you are looking for realistic recordings, with natural sounding LR separation, where the instruments are well separated from left to right, as well as front to back, nothing can beat well recorded classical recordings. They don’t even have to be ’audiophile’ recordings, most standard, major label, classical recordings have this natural sounding LR spatial separation.
Classical recording engineers tend to use what is known as the Decca tree type of microphone setup, which is known to capture much of the ambient cues of the venue where the recording is taking place, usually a concert hall.
A good classical recording, if the system is capable, will reproduce the violin section, which is on the far (audience) left of the orchestra, beyond the outer edges of the speakers, and the double basses, which are on the far (audience) right, beyond the outer edges of the speakers.
Percussion should sound as if it is coming from behind all other instruments, and off to the right.
