IMO the first LP anyone should buy is any ol’ direct-to-disk. Not for the musical content (many of them are not good in that regard), but because of their INCREDIBLE sound quality! On the other hand, doing so will set the bar so high that most other LP’s will thereafter sound mediocre in comparison.
As for good music in good sound, those two criteria are all-too-often mutually exclusive; a lot of my favorite music was unfortunately recorded indifferently or worse. But there are exceptions: A Meeting By The River on Water Lily Records (featuring the playing of guitarist Ry Cooder); Shoot Out The Lights by Richard & Linda Thompson; Where’s The Money by Dan Hicks & His Hot Licks; The Band (aka the brown album) by, duh, The Band; My Life by Iris Dement.
For Classical: Vivaldi’s The Four Seasons by The English Concert directed from the harpsichord by Trevor Pinnock (CRD 1025), or any of his harpsichord recordings on that same UK label; any LP on Harmonia Mundi USA. (lots of Baroque). Holst’s The Planets conducted by Sir Adrian Boult (E.M.I. ASD 2301); Richard Strauss’ Also Sprach Zarathustra with Fritz Reiner conducting The Chicago Symphony Orchestra (RCA); any of the LP’s on the ARK label out of Minnesota, recorded by speaker designer Robert Fulton.