What's a piece of classical music that everyone knows but most people don't know they know?

The Nokia ringtone is a musical phrase from a piece of solo guitar music by Francisco Tárrega, called Gran Vals from 1902.

4'33"

One time I was listening to classical music because I was in a mood. It was a Mozart piece. The piano player started playing Twinkle, Twinkle, Little Star. At first I was like, "if bro is such a genius, why did he rip off Twinkle, Twinkle, Litt- oh, he wrote it."

Fur Elise.

O Fortuna, https://youtube.com/watch?v=GXFSK0ogeg4

Night on Bald Mountain (Mussorgsky), The Planets (Holst), and Ride of the Valkyries (Wagner) are all pretty badass but often get used in movies, game trailers, even ads without being named.

EDIT: Everyone likes links, ja?

In the Hall of the Mountain King

Canon in D, used constantly in modern music and people usually don't recognize it. If you don't believe me go listen to Maroon 5's Memories. I wouldn't blame you if you didn't want to though...

"La Donna è mobile" from Verdi's Rigoletto. Have you seen a pasta sauce commercial? Then you've heard this aria.

Also Sprach Zarathustra. (Thus Spake Zarathustra) very overused, but one of the greatest pieces of music in all of history.

Grieg, Peer Gynt Suite No. 1, Op. 46 "Morning Mood"

Everyone recognizes Erik Satie's Gymnopedie no 1.

I feel like it was just used all over the place, subtly, all our lives. People can rarely name it. Everyone knows it.

Here's a piano version as well

You know Entry of the Gladiators, but you might not think of gladiators when you hear the song.

Vivaldi's The Four Seasons has some pretty recognizable parts in it, such as the first Allegro for Spring, which is popular for fancy receptions, or the Allegro for Winter, which is a great theme for a descent into madness.

Also, Bach's Toccata and Fugue in D Minor, a classic "something evil is about to go down" piece.

William Tell Overture - An entire generation of people came up knowing a portion of the song as the Lone Ranger Theme.

Also, I suspect very few people know The Blue Danube by name, but almost everyone could hum the entire thing if prompted.

The flower duet. Used in countless movies: https://youtu.be/8Qx2lMaMsl8?feature=shared

Aquarium from Carnival des Animaux, Camile Saint-Saëns

https://youtu.be/YVpl-RNzdE4?si=tfZu_ItXehzanh2k

Can-Can, Offenbach

https://youtu.be/4Diu2N8TGKA?si=C3venw8SQQx0vYMF

Russian Dance - Tschaikovsky

https://youtu.be/kgnLGZCyQlk?si=ZA1Of_ViUFPPP6ng

Barber of Seville if you've seen Looney Tunes: https://youtu.be/OloXRhesab0?si=AJ8fNilF8gVtpqsq

Adagio for Strings by Samuel Barber. Usually the "The Hero Dies Tragically" theme.

youtube

Used in Platoon at Elias' death for example.

Not necessarily classical since it came out in the 1940s, but Aram Khachaturian's Sabre Dance (when things go off the rails)

Air on the G String. Although as I said in another comment Bach is technically baroque not classical.

The Radetzky March by Johann Strauss.
It’s used all over the place.

If you know the slogan, “Come fly the friendly skies, you probably know Rhapsody in Blue by Gershwin.

And it’s not classical, but the Wii store theme used to be a variation on Surfboard by Juan Garcia Esquivel

IDK what is considered classical music, but the song that many people know as "Circus music" is actually an orchestral piece called Entry of the Gladiators composed for Czech military parades

This one is a bit newer but you'll recognize it immediately.

https://www.youtube.com/watch?v=qaC0vNLdLvY

Pachelbel's Canon in D. Well, most people probably know it, but its melody is also hugely overused in pop, and turns up way more than most people realise. If you've got five minutes and haven't seen it already, go watch Rob Paravonian's Pachelbel Rant.

Vinheteiro to the rescue!