You know, a thread like this has a lot of elements of personal taste. And on another note, I wish people would give reasons for their nominations.
Beethoven: Symphony No. 9 - Berlin Philharmonic, Herbert von Karajan (Deutsche Grammophon)
I somewhat disagree. For a musical work like Beethoven's 9th, which can mean a lot of different things depending on interpretation, one has to take a survey of the available recordings and decide which one suits your taste the most, or interprets the 9th in a new and different way. That being said, Karajan's 9th (all of them except for the 1963) have a somewhat generic Karajan quality to them. There are much better versions out there. For example, Furtwangler's 1942 recording (in Berlin, during the war) probably tops the list as the most intense rendition ever recorded. It is white hot and so full of angst and danger. Pity that the recording quality is absolute rubbish - it is mono, dynamically compressed, clips, and there is a lot of background noise (including actual Nazis in the audience who were coughing).
If you want something different, Harnoncourt's recording on period instruments is genuinely refreshing and has a lightness, fleetness, and clarity that you won't hear anywhere else. I have said elsewhere that MOST of the clarity of a performance, probably 80%, is down to the conductor and musicians. The other 20% comes from the recording engineer. He just needs NOT to botch it up. Sadly, most DG recordings of the 80's and 90's had that awful murky sound, as if they did not know how to use their new digital equipment. Things got much better in the 00's, but by then DG had tarnished the recorded legacy of so many talented musicians on their payroll. Harnoncourt's period instruments result in a less-heavy orchestral texture and you can listen deeper into the music and hear almost every note that Beethoven wrote, even if it's something lurking in the background.
If you are used to big and brash Beethoven 9ths, the Harnoncourt might sound a bit anaemic in which case I would recommend Klemperer or Bruno Walter. The sound quality isn't as good as modern recordings but they aren't terrible.
Chopin: Nocturnes - Arthur Rubinstein (RCA Victor)
Rachmaninoff: Piano Concerto No. 2 - Vladimir Horowitz, New York Philharmonic, Eugene Ormandy (Columbia Masterworks)
Strongly agree with these two. The Horowitz is a classic, although the Concerto No. 3 is the more famous one.
Beethoven: Piano Sonatas - Alfred Brendel (Philips)
I don't get why anybody likes Brendel. He is about as exciting as watching paint dry. Having said that, he is on many people's list of top performances of Beethoven Piano Sonatas. I would personally recommend Artur Schnabel, who was much more philosophical and has a lovely tone but he has his drawbacks. For one, he is more ambitious than his short, stubby fingers would allow meaning he would attack a passage at a speed that most modern pianists who are more technically accomplished wouldn't dream of doing, then he hits wrong notes, struggles with the tempo and you think that by the end of the accelerando it is all going to fall apart. But he somehow pulls it all together. It is this imperfection of Schnabel that makes his cycle seem more human and endearing. That, and his grand idea of Beethoven and his beautiful tone. The downer is the poor recording quality from the 1920's.
I could go on and on about Beethoven Piano Sonatas (in fact I could write 20 posts on all the different versions), but you would get bored of my ranting so i'll stop by recommending Wilhelm Kempff (a modern day Schnabel), Emil Gilels, Paul Lewis (Brendel-like but more interesting) and Claudio Arrau (probably the most exciting performer I have heard who still plays with taste). There is of course Sviatoslav Richter, but he never recorded a complete cycle of Beethoven sonatas so I am going to leave him out. But it is more than worthwhile to seek out his recordings, of which there are many, and on different labels, and some are better than the others. The attraction of Richter is that he plays like a hypnotic octopus. He seems to have more than 4 limbs, and there is a certain trance-like quality to his music.
Bach: Goldberg Variations - Glenn Gould (Sony Classical)
I hope you mean the 1955 version and not the 1981 one, because to me the 1981 one is unlistenable. Even the 1955 one, which many consider a classic, is borderline unlistenable for me. For a much more insightful recording, try Angela Hewitt 2015. You get the feeling she is trying to get inside Bach's head rather than imposing a historically and academically incorrect interpretation, which is what that Gould is. Both of them.
Schubert: Winterreise - Dietrich Fischer-Dieskau, Gerald Moore (Deutsche Grammophon)
Agree, this recording is a classic. But there are modern variations which are even better, e.g. Ian Bostridge as tenor. I own two of his recordings of this work, one with Andsnes and another with Julius Drake (on DVD), and there are a few other versions extant with other pianists which I have not heard. While Fischer-Dieskau is a baritone and brings a deeply rich quality to the recording, while Moore has his usual chameleon-like quality to change his expression according to the piece, Bostridge has a certain vulnerable quality which brings this piece to another level. He has a small and weak voice, certainly not as powerful as Fischer-Dieskau. But you don't need to have power to be a lieder singer. You need insight, diction, and expression, and Bostridge has that in droves. You might find it hard to believe that someone is more expressive than Fischer-Dieskau but you have to hear it to believe it.