Ok, now his is a tough question. Because ballet music covers, always under the same name of ballet music, more than 350 years, including Lully, Rameau, and other French composers at the cour du roi, the romantic (not only Delibes, Lakmé, Tchaikovsky, but also Beethoven-a good Prometheus music if you ask me!-, the fruitious early 20th, and the modern (Glass, notably).
I luckily had the opportunity to attend a performance of Les Siècles, a wonderful french orchestra, during the summit of the Radio Festival in Montpellier. They played the usual Rameau Indes Galantes Suite, Lully as well, Delibes, Lakmé, and the Rite of Spring. I was quite thrilled by Rameau, and I had the confirmation that Romantic ballet, at least the popular one, is just, musically speaking, sweeties and ruins of delicatessen from a wonderful era. Of course, the Rite is more mind-crushing than any other musical piece - still to this say!-, but in terms of musical beauty, choregraphic poetry, public acceptance, and yet modernity and shivering light, the Firebird seems pretty hard to beat, and Prokofiev's Suite Scythe comes close with those criteras. I know, musically speaking, it's almost Haydn, put together even with Pétrouchka. But the stylistic and formal "gaps" of the Russian Tradition, including Rimsky's heavy basses and codas, are here transcendented by orchestral variety, an attention not to musical form but to the story! Not a "dumbening" ABA form...
But if there is something even greater than this, It's Debussy's Jeux.
Musically, a storm, it does not have the structural and popular appeal of Images, the assumed colorism of Nocturnes, but shares images with Pélléas, it does not have the comfort of La Mer, works and grows by exaltant convulsions. The height of Debussy's genius, modernism, even in the "drama"; no musical rule is here "applicable" (formally quite obviously, but in the little melodic shapes, even the orchestration, Debussy goes WILD and almost INSANE). Plus, the argument is... Well you know, it's one of those little stories, almost anecdotes, that makes the listening of the music absolutely delicious, like Cosi Fan Tutte. "The scene is a garden at dusk; a tennis ball has been lost; a boy and two girls are searching for it. The artificial light of the large electric lamps shedding fantastic rays about them suggests the idea of childish games: they play hide and seek, they try to catch one another, they quarrel, they sulk without cause. The night is warm, the sky is bathed in pale light; they embrace. But the spell is broken by another tennis ball thrown in mischievously by an unknown hand. Surprised and alarmed, the boy and girls disappear into the nocturnal depths of the garden."
The argument is "nothing", everything is about the mood, the atmosphere, a Turner of the tennis court, just three people, and mysteries surrounding, themselves lost ina city. There is no sacrifice, nor a devil to destroy, just an anecdote that could happen to everyone. And at the same time, a vast quantity of images is offered to us. Van Goghian modern images, some kind of non-mystical (but mist-ical!), subtle erotism that everyone can experience, here the flirting of Pélléas an Mélisande seems less exclusive but as orgasmic. Plus, some ideas are really what Debussy was into, generally speaking! A new type of light, a new type of darkness, of night, and yet images of love are here: the moon bathed, embraced by the coulds, lonely lovers surrounded by civilization, still, blessed by the moon (or is it an electric light?^^). And, for Debussy, this is the ultimate Ode to a new time, when, as the nights are lit up by man, and consequently days are not days anymore, love and attraction is to be found and expressed through little gestures, not in "special" moments, not in the middle of a lonely dark night where the lovers, unable to seeing each other, swear solemnly their undying love for half an hour (don't get me wrong, I love Tristan and Isolde), and flirt is now a pleasure to create, seems to be a special sign of attention between those people.
These three people, in the argument of the ballet, are friends. They're young and beautiful.
Let the Games begin.