Gilbert and Sullivan is something people either love or hate. I’m on the “love” side.
There are a few of their operettas that are well-known to everyone (“HMS Pinafore”, “The Mikado”, “Pirates of Penzance”) but one of my absolute favourites of their operettas is the seldom-performed “Princess Ida.”
Here is a song from “Princess Ida” Princess Ida (2003) - Sing Hoity Toity - YouTube
A long time ago, Columbia put out an LP of Charles Ives’ works conducted by Leonard Bernstein. The highlight was the Symphony No. 2, but the B side had a triptych of Ives pieces that just worked so well together played in the order they put them on the LP. It’s still one of my favorite listening experiences. They were:
Central Park in the Dark
Decoration Day
The Unanswered Question.
As an aside, I’ve heard Decoration Day by other orchestras and this specific recording is the only one I’ve ever heard which gets the final chime ring right to give it a real air of pathos at the end.
Prokofiev’s “Lt. Kije” Suite.
Alfred Reed’a Russian Christmas Music. Although the Wikipedia description calls it “one of the most popular and frequently performed pieces of concert band literature.”, I defy you to find anyone who hasn’t performed in concert band who knows about it. I loved it when we put this piece on. It has one of the most awesome climax notes I’ve ever heard. It took me ages to find a professional recording of it.
exapno – in what universe is Dvorak’s “New World Symphony” not famous? Not that I don’t love it, too. But there’s plenty of Dvorak that’s obscure.
Brahms’ String Quintet No. 2 in G major, Opus 111. Especially the first movement starting with the cello solo. It dances and sings, like Johann Strauss (I love the schmaltzy stuff sometimes) but the melody unwinds in true Brahms fashion. Just yummy.
I am trying to remember which recording was my favorite, but I drawing a blank.
OMG, yes. It does make me want to invoke a deity, if you know what I mean. 
Vivaldi’s take on La Folia just jives with me.
And there I was thinking it was based on the Laura Branigan song.
Did he use it to work the ped… Oh, never mind. Much too easy.
I am not generally a fan of orchestral music, but I very much enjoy Aaron Copland’s Concerto for Clarinet, Strings and Harp, which was commissioned and best played by Benny Goodman.
Alan Hovhanness’ Mysterious Mountain and, as usual, *Dixit Dominus by that Handel Guy everyone thinks is so hot. For my money, Dixit Dominus beats up the Hallelujah Chorus *and takes its lunch money.
Oh, and Eric Satie’s Trois Gymnopedies
Aaron Copland’s Concerto for Clarinet and Strings. Sometimes makes me cry.
Debussy’s “Lent Melancolique Et Doux” from Images (Oubliees) I. I love the sinister passage at 1:50, and how it repeats later in a major key and sounds so wistful.
Ravel’s “Le Gibet” from Gaspard de la Nuit. That B flat that repeats throughout the entire piece is represents the bell tolling for the hanged man.
Ravel’s 2nd Movement from the Concerto in G Major (speaks for itself :))
One that I used to play as a child: Schumann’s “Almost Too Serious” from his Kinderszenen
This isn’t especially obscure, but I had never heard Pergolesi’s Stabat Mater before I got my Spotify subscription. I particularly recommend the Concerto Italiano version.
I honestly don’t know if this is “super famous” or not, but I had never heard it until last year. Now I can’t seem to listen to it enough:
Arvo Part - Cantus in Memoriam to Benjamin Britten
(Hi Qad!: I love Lt. Kije!)