OK, I’m not actually going to pick a song and say it’s the most beautiful; but if I asked someone else that question and they answered Nessun Dorma, I’d accept that as a defensible choice.
Nessun Dorma is the tenor aria from Turandot reawakened in public attention a few years ago by Paul Potts on Britain’s Got Talent and earlier by Pavarotti at the 1990 World Cup. Everybody’s sung it. I’m not going for a definitive analysis of a well-studied aria; I’m calling attention to some points that I’ll reference later in connection with other songs that I’d entertain as good answers to the question.
What is it that makes the key parts of this song so wonderful? When you break down the melody itself, it’s mostly simple. Most listeners don’t speak Italian. It’s usually heard out of context. After eliminating all that, I’m left with the use of the tenor voice, the orchestral accompaniment, and a few meager thoughts on the melody.
It’s all about the shape of the phrases. Ignoring the words, plot and characters from the opera, I’d conclude it was a song about climbing, reaching, holding, standing. The first part–the A section, if you will–has the tenor pushing to the major seventh (“o, Principessa”; “guardi le stelle”). Then in the more recognizable B section, it’s a rising and falling scalar figure. What makes it work for me is the motion to inversions in the bass, which along with strings doubling the melody manages to suggest parallel sixths that aren’t really there (“Ma il mio mistero è chiuso in me, il nome mio nessun saprà!”) culminating in an opposing figure that takes the V chord to a first inversion of the (momentary) tonic with the melody holding on the high fifth (“sulla tua bocca”). It’s just a ii-V7-I, no extensions to the chords and hardly a passing tone to be found; but I could name that tune in one note if it was that note, and I wouldn’t even confuse it with Mussorgsky’s “Great Gate of Kiev” (which sits itself characteristically on that same inversion).
However, I can pick out three specific chord-melody interactions that I think are important, and I’ll make it a little more clear why in later posts on other melodies. In order of impact, at the end of the B section, it resolves not to the tonic but to the subdominant (applying those terms as if the piece were in D rather than G) ; in the A section, there’s a substitution very like a jazz altered dominant 7; and near the end of the B section where the melody is a simple descending D major arpeggio, it changes to the Gmaj7 on the F#.
The change from first inversion D to Gmaj7 is simply letting the leading tone in the bass drive the chord change. The altered substitution uses an F# against the D in the melody to suggest the V, has a Bb to suggest the minor–I’d almost call it a blue note–and puts an Eb in the bass. An altered D would be D-F#-Bb-C-F, and here we have D-F#-Bb-C-Eb; with the Eb and Bb in the bass, it sounds like an extended Eb minor, and it’s essentially a G harmonic minor scale, but it fills the role of a D7. The resolution to G is a continuation from the inversion: D/F# – Gmaj7 – D/A – A, and to G instead of D; that’s followed by the altered chord at the top of the A section, which re-establishes G as the key.
Enough armchair analysis; what do I take from it? First, that one way to get a strong circular motion between an A and B section is to have a bimodal key center in fourths. Second, that if you go from the tonic to another chord and back again, with most of the time on the tonic, that transition chord–whatever it may be–can be substituted for the dominant elsewhere in the piece. Third, that the melody doesn’t have to dictate the chord changes, especially when the bass has good ideas on where to go.