One thing I've noticed in the thread is the lack of mention of performers. Performers are as important as composers, especially if the piece leaves room for a lot of different interpretations.
I don't know much relatively about symphonic music, but I consider Furtwangler to be one of the greatest if not the greatest. His main repertoire was the core German one: Beethoven, Brahms, Schubert, and Bruckner. He aimed not to just follow the composer's written notes as so many conductors do, but to achieve a transcendental experience through music. I also highly recommend Klemperer who had pretty much the same repertoire.
Of violinists, there are so many great ones but here's the briefest of overviews:
Jascha Heifetz - Unquestionably one of the greatest if not the greatest. Think of him as the person who achieved a 3-minute mile. Although many were/are on his standard both technically and musically, he was revolutionary at the time. His most famous hallmarks are his immediately identifiable silvery, highly-charged tone and ultra-human technique. He was greatest in the Romantic repertoire, especially the Brahms, Tchaikovsky, and Bruch concerti. One of my all-time favorite recordings is the Vitali Chaconne by him, and it perfectly illustrates his playing. He's universally regarded as one of the gold standards. As Gitlis said, "You can like, not like, agree, disagree, but Heifetz was unique, there's no doubt." Critics often accused him of coldness in his playing due to his conservative stage demeanor, a laughable accusation when you actually listen to his records. Personally, stage manner is one of my main complaints against today's classical music scene, but that's a discussion for another day.
David Oistrakh - Like Heifetz, also regarded universally as one of the greatest if not the greatest. Famous for a distinctive warm tone, he prefers a slower tempi than most which is why I think of his playing as introspective. He had an unfailing technique and musicianship of the highest order. Like Heifetz, he's known for his interpretations of the Romantic repertoire, but he was also very successful in chamber music and violin sonatas, whereas Heifetz was below his own Heifetzian standards due to his tendency to dominate instead of cooperate. His every note and phrase is well thought-out and crafted, but not mechanical or over-rehearsed.
Nathan Milstein - Famous for his extremely clean articulation and no-fuss way of playing. He had a technique equal to Heifetz (I think he, along with Heifetz and Ferras, has a very distinct way of holding the violin and very unconventional technique that suited him) and I think of his tone as "plain" without any of the negative connotations; very direct with a slight edge to it. He was famous for the same Romantic repertoire as Heifetz and Oistrakh, but he's even more famous for his Bach of which he was an undisputed master. Any discussion of Bach's violin music inevitably mentions Milstein. He, Grumiaux, and Szeryng were the three in the "old" days whose Bach was most admired. With an impeccable musicianship and cleanest technique, he had the tools to play Bach ideally.
Arthur Grumiaux - He's the most well-rounded violinist of his generation IMO, along with Henryk Szeryng. He was widely admired for his music in all three periods - Baroque, Classical, and Romantic. His recordings of Bach are often reference recordings (his recordings were on the Voyager), his Mozart concerti hold the same status, and he was fiery with a twinge of his classicism. I often think of his tone as very aristocratic, refined, and reserved, just like French wine. He's very under-appreciated IMO, probably because he had a mostly European-based career and he recorded for Phillips.
There are of course, many many others. Kreisler, whose playing was the most charming and sweet of them all. Ferras, Neveu, Kogan, Szigeti (whose technique was failing him in his later recordings, but possessed the highest musicianship), Francescatti, Enescu, Menuhin, Perlman. All of them were wonderful musicians.
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