For sonic beauty, I recommend Karajan's phenomenal Bruckner 8 on DG. IMHO, this is one of the finest classical recordings ever made. If you like Bruckner, the Tintner recordings on Naxos are also very good, with excellent sound quality. Other good picks on Naxos are the Bach Motets and the Dvorak and Janacek string quartets. If you like guitar music, some of the discs in Naxos' Laureate Series -- featuring young artists, usually winners of major competitions -- are really great. I would recommend particularly the recital discs of Antigoni Goni, Denis Azabagic, and Franco Platino. These are all recorded in a massive old church in Canada, I believe, and have exceptionally clean sound. EMI red line has loads of terrific older recordings that are very cheap, but sound quality is of course mixed. You might check out Barenboim's recordings of the Brahms piano concertos and Gidon Kremer's of the Sibelius violin concerto.
Hope this helps, JJ.