Tall order as there are thousands. I'd suggest buying the DGG set celebrating James Levine's 60th B'day. The 3 CD's contains spectacular performances of Brahms, Stravinsky, Bartok, Sibelius, Beethoven... Uniformly stunning. Next I would get John Eliot Gardiner's recording of Bach's St. Matthew Passion (or Suzuki's recording of the Matthew Passion on the BIS label), Beethoven's MISSA SOLEMNIS (Gardiner) and the Beethoven Nine Symphonies with Abbado conducting the Berlin Philharmonic. After that some random suggestions: CHRONOCHROMIE of Messiaen (Boulez conducting on DGG), Boulez's DGG recordings of DEBUSSY with the Cleveland Orchestra, Schoenberg's GURRELIEDER (either the Chailly recording on Decca or Abbado's recording on DGG)....
