I prefer Yevgeny Kissin's performance of Rachmaninoff's 2nd and 3rd concertos. Argerich performance of 3rd, last movement, sounds like very forced playing and tempo is too fast. To me it sounded like she was pushing it past her abilities. Technical skill required here is by all acounts very high. Another recording considered legendary is by Horowitz. This one has the first movement in VERY slow motion however. The orchestra and conductor (Ormandy) are the same that recorded the same piece with Rachmaninoff himself.
I double the recommendation of Beethoven's quartets by Alan Berg quartet. The last movement of Op. 131 is incredible and among the very best of over 300 CDs that I have. I've discovered it in my collection two months ago and was so impressed that I got the whole set recently. Later I've read in notes that some people consider that quartet as Beethoven's best work. I can see why.
I really like Bernstein's performance of Mahler's 2nd. Those who love Mahler might want to investigate Gilbert Kaplan's recording of this work as well.
One work that I've discovered few years ago and can't believe isn't more popular is A Colour Symphony from Arthur Bliss. I can't find recordings easy enough, so I have only two. Hyperion one is I think much better than Naxos but the Naxos includes Adam Zero ballet which I heard only once but has a great promise. At any rate, Colour Symphony is a masterpiece that shouldn't be missed.
Prokofiev's 7th symphony is great as well. But I haven't found a CD performance that sounds good yet. Only a USSR-made LP so far have mainained the beauty of this piece. However, you CAN find 5th symphony recorded recently by Ashkenazy (?) and a German orchestra (the same one Kubelik used to conduct, can't remember the name), and it will blow you away. I had to copy it from local library as it doesn't seem to be in stores (I even had trouble making salesguy believe me that it exists ). Stunning.
Also, some people here recommended an incredibly skilled pianist that seems to like to record unknown pieces (can't seem to be able to remember any names today, sorry). The one with works of Alkan contains not only great, forgotten music but also has high recording quality.
Karajan's collection of Tchaikowsky's symphonies (all 6 of them, recordings from 1970's) is also worth the money.
Hmm, maybe I should dust off those Bartok's String Quartets and listen to them
. I also have the Emerson Q. recording that I never listen to. Getting into more avantgarde 20th century music requires time, patience and it still does not guarantee satisfaction in the end.
Btw, Hindemith's symphonies (I believe my recording is by Maris Janssons, heavily misspelled) and Pacific 321 are also very nice, if you can handle 20th century music (this one is not that bad actually).