What 50s Classical Mono LPs to cull?

I’ve come across a huge collection (hundreds) of late 50s era classical mono LPs in excellent shape both vinyl & jackets. While I’m sure artist/performance is the key to keepers in this lot (for instance Verdi’s Requiuem Munch/BSO/RCA in leather binding), are there generally any labels that were consistant in top fidelity form between ’57 – ’60?

The collection consists largely of Opera but has an excellent showing in early classical. The labels are London/Decca – RCA – Angel - Everest – DGG as well as a few Lira and Melodiya (unpartnered). Maybe the question should be – any of these labels to avoid?
Thanks for any help in advance.

- Mario
Go to the Absolute Sound and Stereophile web sites for their Records to Die For lists. Thats a start. Some of the RCA Red Dogs (Red Labels Stereo) are coveted and the Gray Dogs(MONO Gray label) are too.
Send them to me and I will get rid of the bad ones...
The two LPs left, I will return to you.