Yourmusic is a veritable goldmine of Classical. As a starter, in their new releases you can find the Prokofiev Complete Symphonies (Gergiev/London Symphony Orchestra) which will disappear if you don't get them now and the Nelson Freire/Riccardo Chailly Brahms Piano concertos. You should definitely get those while they are available!
Another set that you should consider are the Beethoven string quartets (3 box sets) by the Takács Quartet. Just put Takács Quartet into the search box for artist and they will come up. Other great sets are the Gardiner Beethoven Symphonies, the Bruno Walter Original Jacket collection which consists of symphonies by Mahler and Bruckner, and by all means look at the Emerson Quartet's Schubert Quartets and Octet. You will have to do an artist search on John Eliot Gardiner, Bruno Walter and Emerson Quartet to bring them up. And Leonard Bernstein has the Beethoven symphonies as well. Avoid the Rattle Beethoven it is not particularly good and the only Mahler by him that I would recommend at yourmusic is the Symphony 10 (Cooke completion) and possibly the Mahler 5th, but that's not the gretest performance that you can find around. Avoid his Schubert symphony as it's very mediocre and might turn you off to Schubert entirely.
Anne-Sophie Mutter's Beethoven String Quartets is a great addition as well.
Charles Koechlin's Chamber Music for Flute and Clarinet music are also excellent (put Koechlin into the composer search).
For Bach keyboard music, they are offering the Christophe Rousset box set which includes his Goldberg Variations (out of print for so many years, they sold used for 2 or 3 times the price of this newly remastered set), Italian Suites, Overtures, Partitas, et al. It's a real bargain but be aware that it's harpsichord music and you may have to grow into it.
They also have an SACD/Hybrid of Chopin by Arthur Rubinstein (artist search) and another Chopin and a Beethoven sonata recording as well a DG recording of the named Beethoven sonatas by Wilhelm Kempf.
Artist search on Nikolaus Harnoncourt and you will find the Mozart Requiem, Bruckner Symphony No. 5 and some Dvorak as well that are all excellent. Artist search on Thomas Fey for Haydn Symphonies (some of the Paris and Sturm und Drang symphonies). Fey's Haydn is excellent. Artist search on Colin Davis for Sibelius symphonies and Sibelius's tone poem Kullervo. There are also his Dvorak works, the symphony no. 6, 7 and Sarah Chang's recording with him of the Dvorak violin concerto.
Artist Search Leif Ove Andsnes for Schubert, Mozart, Rachmaninoff piano concertos and especially the Grieg. Avoid his Haydn -- it's not particularly good, though.
For Tchaikovsky, there are a number of recordings of the 1812 overture (overplayed and overvalued chestnut that it is, it's still a crossover pleaser) but you will also find the SACD/hybrid of Van Cliburn's recording of the Piano concerto no. 1 and the Rachmaninoff Piano Concerto No. 2, and that is a great sounding historic recording with performances of both works that are stellar. Also from Tchaikovsky, look for Antal Dorati's SACD/hybrid of the Nutcracker. It's one of the best interpretations of the famous ballet.
Again in the boxed sets, don't miss the Stokowsky Decca Recordings or the Bach Transcriptions.
Also, look for Vladimir Horowitz's recording of Domenico Scarlatti (although if you get the box set of Horowitz, I believe they are included in the set).
Artist search Andreas Scholl and Thomas Quasthoff for amazing Bach, Mahler and baroque vocal music.
Artist search Giuliano Carmignola for Venetian baroque music.
Artist search Carmignola for more Vivaldi or Cecilia Bartolli.
I find anything by Concerto Köln is usually very interesting.
One note of caution: they still have the George Szell original jackets (Beethoven's Symphonies and orchestral music) which is expensive for what it is. A better value for about the same price is the Beethoven symphony and piano concerto set at
Amazon
. Which will give you the complete symphonies and piano concertos with Leon Fleisher for about the same price. That set is probably one of the best values around and has some of the best interpretations of the symphonies and concertos ever recorded. Similar values can also be found at Berkshirerecordoutlet.com which is the happy hunting ground for recordings that are scarce and recently out of print.