Help with Classical Music Record Labels.

Don't overlook Sony.  All the great old stuff is on the Sony label.  Or so it seems.

Cheers

Command, Westmintster, Nonesuch, Lyrita, just to name a few.

Are you talking vinyl? DG is very inconsistent, 1980’s digital are very poor recordings.
Decca, London (Decca USA releases), Philips, EMI, RCA Red Seal, Mercury Living Presence, RCA Living Stereo, Telarc (digital releases). Columbia which was bought by Sony, L’Oiseau-Lyre.

Worst... Everest, Columbia House, Angel (EMI budget releases), Seraphim (Angel reissues).

Wow, everybody's all over the place. So I'll add my personal take. BTW, I get enjoyable fidelity from all formats -- vinyl, CD & streaming.  I've also been listening to classical music since the age of three.  I'm now, ahem, in distinguished retirement.

Best sound quality overall for me has been on Philips. The Concertgebouw Orchestra and the Academy of St. Martin-in-the-Fields almost never fail to give me musical and aural pleasure. Some call the St. Martin-in-the-Fields performances bland. I don't.  Saw 'em live a couple times, too.

Telarc can often be a bit too analytical, but when it is good, for example in the Lauren Maazel recording of Tchaikovsky's Fourth Symphony, it is very good.

RCA Living Presence, yes even in the CD/SACD reissues, are audiophile go-to's for good reason.  Slaughter on Tenth Avenue with Fiedler and the Boston Pops is a standout.

CBS/Columbia can get all too trebly, especially with Bernstein and the New York Phil, but the performances can be persuasive.  The Bruno Walter recordings from the 1950's, especially a transcendent performance of the Schubert Ninth Symphony I've got on both Odyssey and a Japanese pressing, are always worth a try. So is any recording with Murray Perahia.  

DG is sometimes unbearably trebly, but that doesn't keep me from having them regularly on the turntable (my DG CD's, unfortunately, usually show DG at its harsh worst).  My DG LP recording of Bizet's Carmen (conducted by Claudio Abbado and featuring Placido Domingo) might be my single favorite album in any genre. Drama! Color! Tunes! Chaotic 3D crowd scenes!