From the moment the opening chords of "One" hit your ears, you will probably realize that you are about to hear something powerful.
Like the lyrics, the music for "One" is deceptively simple. "What this song is," said The Edge, "is very clear, very straightforward. Clean guitar and keyboard, Bono's great lyrics, and a great melody."
Clean, crisp, and clear, indeed. There is not a ton of feedback or layering of sounds in the background. The individual instruments, as well as Bono's distinctive voice, all stand out individually from one another, even while blending to form a perfect melody. As Bono's sings, we are all individuals but we must come together in order to make peace with each other—in this case, to play beautiful music. It almost sounds as if he is speaking directly to his bandmates, saying, "Hey, we are all great musicians here, we all have our unique instruments and unique talents, so let's combine them to make something truly spectacular."
"One" begins in the key of A minor, creating a mood that perfectly fits the lyrics. (A minor is the relative minor of C Major, the most basic key around—it's the Do-Re-Mi key. A minor, like C Major, has no sharps or flats, but since it is a minor key, it is decidedly less cheerful, and much more brooding than its cousin. Like C Major, it is very simple to play, yet incredibly rich and melancholy.) It perfectly fits the message of the song, which is that we must try to get along with one another even when it seems impossible—a concept more bittersweet than joyful.
Next, the song switches to D suspended, F Major, and G Major. These major keys are all built on notes that fall within the A minor scale, so they naturally sound good with it. They also add an uplifting note to the music, implying that love, like anything else, is a roller coaster of highs and lows and never stays the same for long.
The chorus is where things really get musically interesting. When Bono sings "One love," he switches to C Major, but then "One life" goes right back to A minor. "One need" is F Major, "In the night" is back to C Major. So the chorus is in two keys, not just one. F Major and G Major belong in C Major as well, but D suspended belongs to neither. It is very rare for a musician to write a song that flips back and forth between relative major and minor keys, using the same sharps and flats (in this case, none). Usually a song will jump to the fourth or fifth key in the original scale and form a new key from that, so what U2 did here is very unusual. It produces a rich, melodic texture that is very unlike the run-of-the-mill pop that we usually hear.
If all this talk of sharps and flats and Majors and minors is Greek to you, let's put it this way: "One" just sounds good. Like any beautiful work of art, it's hard to articulate exactly why we like it; we just do. The human brain's ability to appreciate art, music, and beauty is one of the greatest scientific mysteries, and each person's experience of art is unique.
In 2003, the British magazine Q asked a group of musicians to vote for what they thought was the greatest song, and "One" made it to #1. When asked why he thought this happened, Q's editor replied, "I think people voted for this because it's a classic that still feels fresh… It's a great tune but despite being over ten years old we haven't had time to get bored of it… It's inspirational, it makes people feel good, but it also feels profound." (Source)