I have never liked his piano sonatas, unlike his smaller scale works for piano solo. I am however in love with his concertos. His piano sonatas, while also larger works just cannot be compared with his concertos, in my opinion.
Sonata no. 1, I think, is a masterpiece, although I'll agree that I prefer his concertos. I have 6 different recordings of his third (Sergei himself, Earl Wild, Vladimir Horowitz with the New York Philharmonic, David Helfgott, Evgeny Kissin, and Martha Argerich). I have to force myself not to listen to his third too often because I do not want to wear it out. I really like the first movement of his first, but the second and third movements do not appeal to me, and his second concerto is also phenomenal. I haven't listened to his fourth more than once or twice and I don't remember it at all--I'm saving it for a rainy day

I'm surprised by just how consistently good Rachmaninoff's piano works are. His preludes are all at least good, and his etudes-tableaux are sensational!
I really like Isle of the Dead, but I don't like most of his symphonic works...violins hurt my ears!
I'm a huge Rachmaninoff fanboy, but not quite enough to have used him in my username (looking at you, Rachmaninoff_forever...

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