best classical LP labels

Depends what you mean by "best", but if you're basically asking what would we pick as a ONE "desert island" label -

I would have to agree (and expand) Sean's pick by saying Decca/London. Their catalog is huge and varied, and the sonics are almost 100% between very good and outstanding depending on the specific recording and pressing.

My next "desert island" choice - if Sean already had taken the Decca/London's to his island - would be Philips. Also a great catalog with generally very good sonics. Not nearly as many in the "outstanding" category, but predictable with very few clinkers.

3rd choice might surprise you - MHS - that's right - Musical Heritage Society. If audioSnob appeal means little to you, this is a fantastic catalog of both superbly remastered AND pressed reissues from all over the world, as well as MHS's own recordings which are really top notch - especially the earlier ones done under Kurt List. Their later Digital recordings and thoe mastered by Bill Kipper are also excellent. I would much rather have their catalog on a desert island than say, Mercury or RCA. MHS is the best-kept semi-secret in the Classical Vinyl universe.

re: Mercury Living Presence and RCA Living Stereo - they do have some really spectacular recordings which are must-haves, but IMO Many, many of them are overrated and/or very "pressing specific" in my book. I have had many Living Presence lp's that outright sucked. Also, the size and variety of their classical catalogs, mostly centered on large scale orchestral, doesn't compare with the above three. Unless you have a cheap source for the originals of these, I would stick to high quality reissue versions like those from Classic Records which are already handpicked for their underlying content.

There are many boutique or smaller labels that are often superb sonically and performance-wise such as Harmonia-Mundi France, Lyrita, Claves, Finlandia, Proprius, Wergo, Telarc, etc., etc.

Both the early Decca (rarer big print Decca label) and the early EMI cream and gold label are just about the most natural and finest sounding classical records you will ever find. The Deccas produced in Germany fall far short soundwise compared to the British pressings. A number of the Mercuries are overrated, both in sound and performance. Some of them tend to have a little too much treble emphasis, and might be a bit lacking in warmth. Invariably, the earliest Mercuries, typed lettered FR-1,FR-2,etc., are superior sonically to the later hand inscribed RFR-1, RFR-2, etc. Also, as a conductor, Antal Dorati was not in the same league with Reiner, Klemperer or Beecham. Some of the earlier RCA shaded AND white dogs were also very good, however, all of those are not necessarily superb in sound. I just don't have the desire to go through a "grading" list at this moment. Someone else mentioned the Philips discs. Yes, a good number of the earlier (and thicker) pressings sound very nice. Avoid the much later, extremely thin "floppy disc" Philips lps. And here's the sleeper: The made in England London Stereo Treasury series recordings are typically very fine. I fail to understand why they continue to be ignored by classical record lovers, especially since they can still be gotten relatively easily and at much lower prices than the big "collector's items". But make sure that when you buy a London Stereo Treasury pressing that it is NOT the later, lousy U.S. cream, red and blue label. There are also some pretty good sounding Columbia 6 eyes, with some of the finest performances ever put on disc by conductors like Bruno Walter, Eugene Ormandy, and yes, Leonard Bernstein, whose stature in my eyes and ears over the years, has grown considerably.....Have a good time searching, and good listening !

My cents re: Columbia and DG

I find most Columbia 6 eyes and many 2 eyes to be enjoyable recordings and performances. Even the Eugene Ormandy/Phil Orch. which has little collector value can be great listening.
Then there are the Bruno Walter, Glenn Gould, and Budapest Quartet recordings - all very nice.

DG - agree with the others above. More Miss than Hit sonically, but it's a huge catalog with many great performances - and some of the "Hits" are fantastic, e.g. most of Carlos Kleiber's recordings, Bernstein's Mahler symphonies, Schneiderhan's violin performances, etc.
The "Red Stereo" Large Tulip label versions (if they exist) are much better sonically than the later re-pressings. Surprisingly, I find many of the DG Digital Recording lp's are quite good.