I guess this will also sort of show how wide the range of my musical preferences lies.
Purcel: "Dido's Lament" from "Dido and Aeneus" (the whole opera is gorgeous)
Bach: B minor Mass, any part of it.
Mozart: "Le Nozze de Figaro", again, pretty much any part
Mozart: "Zauberflaute", ditto
There's not much in the 19th century that I can really say I like, at least, not until the end of the 19th century, where you'll find the following...
Rimsky-Korsakov: almost all his operas
Borodin: "Knyas Igar"
Mussorgsky: "Boris Godounov", ALL of this magnificent opera (though I prefer Rimsky-Korsakov's version of it. He re-orchestrated it, and made some cuts here and there, which make it stand better). If you do NOT know this opera, it is a monument and you NEED to know it. If nothing at all, at least listen to the coronation scene, in the Rimsky-Korsakov version. It is absolutely TERRIFYING.
Puccini: every. single. note. My personal favourites are the final aria of Suor Angelica, and Liu's two arias in the final act of Turandot (technically, Turandot is a 20th century work, having been composed very shortly before the composer's death)
Then we come to the 20th century...
Hindemith: "Mathis der Maler" (gorgeous opera. a bit austere, but still beautiful)
Hindemith (again): "A Requiem for Those We Love" (ALL of this magnificent and ingenious score)
Ravel: "L'Enfant et les sortilèges", in particular the Fire aria, the child's solo (le Cœur de la rose), and the beautiful fugue that finishes off the opera.
Debussy: "Pelléas et Melisande".... ok, this is a special case. The opera itself is duller than dishwater. You really HOPE the characters would hurry up and die, already. But dammit, the music is gorgeous.
A bit closer to our time...
Goldenthal: "Fire, Water, Paper, A Vietnam Oratorio". I'm partial to the 2nd movement "Scherzo".
And maybe a tad outside the "box" of classical music, though if you study it closely you realize it isn't really that "non classical"
Sondheim: "Into the Woods" (the film version is quite well done, intelligent cuts, pretty good performances by non-singers)
Sondheim: "Sweeney Todd" (do NOT make the unforgivable error of watching the film.. it is absolutely HORRIBLE)
Sondheim: "Sunday in the Park with George" an absolute masterpiece (which won the Pulitzer prize for music), and it speaks directly to the heart of "what is art, what is an artist, what does an artist strive for".
Sondheim: "Passion". A weird story, but oh such beautiful music.
Sondheim: "Pacific Overtures". There's nothing to be said about this wonderful piece, other than that it is wonderful.
And cheesy me, sorry, I LOVE Disney's "Frozen". I know, I'm such a 12yr old girl at heart.