Which classical piece of music is the most well known

[André]

Bach's Toccata and Fugue in d minor has the most well-known first 30 seconds of anything (maybe).

[Daverz]

Either the opening of Boccherini's "Minuet" (from his String Quintet in E major, Op. 13, No. 5, G275) or the opening of Mozart's Eine Kleine Nachtmusik is used in nearly every TV/film scene at a ritzy dinner party.

Grieg's Morning Mood from Peer Gynt is familar from old cartoons.

[(poco) Sforzando]

The Moonlight Sonata, and the first 4 notes of Beethoven's 5th.

[LKB]

Why can't they listen to Stockhausen like normal people?

    ...

[Pizzicato-Polka]

Marche funèbre by Chopin has to be up there.

But the first thing that came to mind is absolutely Beethoven's Symphony no 5.

[Symphonic Addict]

Most of the most famous pieces have appeared already, no doubt about it. However, two other pieces that haven't been mentioned yet, but deserve to be included: Hallelujah from Händel's Messiah and Can-Can from Offenbach's Orphée aux enfers.

[Symphonic Addict]

Part of the slow movement from Mozart's Piano Concerto No. 21 (the one related to Elvira Madigan) is another instance of famous/popular classical music that springs to my mind.

[relm1]

Also Sprach Zarathustra's sunrise opening.