My absolute favorite is 48/1 in Cm. To me, it's the most beautiful of all Chopin's nocturnes, along with being the most difficult to play. Very much a mini-ballade, as far as I am concerned, with all the elements of a grand, poetic drama written for the piano.
15/3 wins the award for easiest difficulty-to-beauty ratio...in other words, it's fairly easy to play, but exudes a great deal of emotion. Who couldn't love that beautiful, melancholic, melodic line which begins the piece? What comes next is surprisingly experimental, with no return to the delicate, tragically nostalgic opening theme.
27/1 in C#m is epic and powerful, with a mysterious theme and turbulent inner section. In some ways, it reminds me of Chopin's scherzos with its haphazard dynamic changes and impulsive bursts of energy. The changes of mood are also quite interesting.
15/1 starts out peaceful and innocent but erupts into emotional turmoil before returning again to tranquility. It is common for people to use weather analogies when describing this piece, but I feel like there is something far more profound being illustrated. The piece does resolve from its frightening state, but there is a moment before the final resolution when the listener isn't sure whether it will once again break into unbridled, nightmarish bliss. To me, the outburst represents an unexpected, extremely emotional struggle or conflict...and the way the piece is composed communicates that these conflicts may happen at any time and true tranquility is only ever an illusion, for the inevitable tragedy will always come. In some ways, I think that at least half of Chopin's nocturnes expel a similar meaning, with their pattern of bookending a nightmarish middle section with calm, dreamlike starting and ending sections.
27/2 is a work of surprising nuance and complexity. It changes from contentedness to longing to suffocation and anguish. There is enough subtlety, both melodic, harmonic and rhythmic to make this one of the most difficult nocturnes to play well. This nocturne illustrates the frighteningly slow, seamless transition from pleasant to tense. There are no defined "sections" as in most of the preludes, but rather, the piece flows fluidly from one thought to another--a continuous stream of emotion and organic human sentiment. Truly beautiful.
C# minor posthumous has one of the most feverish, stunningly beautiful themes of all the nocturnes. The runs in this piece are sweeping and brilliant, showcasing the improvisatory nature of Chopin's compositional style. Not remarkably complex, but beautiful all the same, with wonderful singing for the piano.
They're all wonderful, but these stand out to me as true masterpieces.