best classical LP labels

Mercurys are widely cherished. If you change the question to "make", then Speakers Corner reissues have got to be near the top of the list.
Rushton is the guy whose answer here should be worth reading...
Cheers,
Spencer

Some of this is a matter of taste. The reason I say this is that different labels have a different characteristic sound. I personally like some of the older Decca recordings because the recordings capture the ambience of the venue. The resulting sound is a little more "confused" than say a Mercury, or a DG, but in my opinion it sounds more "live" as a result.

I have some older Decca "ffrr" recordings that I think sound terrific, but others may find them a little too romantic sounding.

There are also the RCA "Shaded Dogs" from the '50s. This catalog contains some amazing performances that were captured by two amazing recording engineers.

It's funny, for a couple of pieces my reference for a LIVE performance is how it compares to the arrangement and pacing of the recorded performance of Reiner with the CSO or Munch with the BSO.

RCA Living Stereo - shaded dog pressings; Mercury Living Presence - FR/RFR pressings; Decca/London/Argo - blue back covers; Columbia - 6 eye pressings; London Phase 4 Stereo; EMI original pressings engineered by Christopher Parker + EMI Speakers Corner reissues; Lyrita (19/20th century British classical music - not to everyone's taste); pre-digital Hyperion; French Harmonia Mundi.