2001: A Space Odyssey – “Blue Danube” (J. Strauss) and “Also Sprach Zarathustra” (R. Strauss). Both are now iconic.
Excalibur – “Siegfried’s Funeral” (R. Wagner). I used to think I loved this movie; it turns out I really just love this music. It’s used throughout, but especially at the end, as Percival returns the sword to the lake.
The King’s Speech – Beethoven 7th Symphony, 2nd Movement. Background as the king delivers his speech. Some of my favorite music in a favorite movie at a climactic moment – goosebump-y.
Honorable mention but ineligible: “Ride of the Valkyries” (Wagner, again) in Apocalypse Now, because it’s part of the movie action, not really on the soundtrack per se.
Would both “Fantasia” movies be disqualified? If not, Stravinsky’s, “Firebird,” segment is mine.
Slight sidetrack: Back in the '80s (and probably '90s, too), it seemed every other movie trailer was using Holst’s, “Mars,” playing over the 1750 2-millisecond clips of movie that made up the ad. But I can’t recall it ever being used in the movie itself.
The OP already nabbed, “Excalibur.” “Carmina Burana,” was used to great effect, as well.
1850 is too early – we’d lose Tchaikovsky, Mahler, Rachmaninoff and Stravinsky. As a definition of what the OP (ie, me) is looking for: if you could walk into Tower Records before the movie opened, go to their “Classical”* section and purchase the music that would be on the movie soundtrack.
*in other words: punt. We can define “classical” in another thread.
I’m 99% sure that this music is the same as the music used in 2001, during the light-show psychedelic ending sequence. Perusing the 2001 soundtrack, it’s one of the tracks by composer Gyorgi Ligeti, composed in the early 1960’s. It may qualify as “classical” now, but in the context of 2001’s premier it’s practically contemporary.