Short answer: I think it depends on the melody and the chords, because key changes often sound seamless when the transition re-purposes the existing melody notes.
Long answer: key changes are often implemented using a transition/lead-in chord. For example, if you're transitioning from the key of Cmaj
to the key of D♭maj
, you might add a V chord (an A♭7
chord) before moving to the new D♭maj
tonal center. The A♭7
would signify a coming key change and prepare the listener for the change.
I'll use London Bridge as an example. When played in the key of Cmaj
, this song ends on the notes E
-C
(played over the word "la-dy" and over the chord Cmaj
). But the notes E
-C
are also found in the chord C7
, which can act as a V chord for a new tonal center of Fmaj
. So you could modify the final chord progression in the following way:
Lyrics: my fair la-dy ... London
Original chords: G7 G7 Cmaj ... Cmaj
New chords: G7 G7 C7 ... Fmaj
And now the C7
chord is acting as a V chord for a new key center of F maj
. In this case, you're moving up a fourth, and it would sound extremely natural--a good transition.
Alternatively, we could recognize that the final melodic notes E
-C
also appear in the chord Amin
. So you could take the Amin
and turn it into the ii chord of a ii-V-I leading to a new key center of Gmaj
:
Lyrics: my fair la-dy ... London
Original chords: G7 G7 Cmaj ... Cmaj
New chords: G7 G7 Amin D7 ... Gmaj
In addition to the melody, you can use harmonic patterns to modulate. For example, it's extremely common for modulations to move the tonal center up a half step. This can happen once or repeatedly, always modulating up a half step from the prior key. For example, if you're playing in the key of Cmaj
, this would entail modulating up to the key of D♭maj
. In the song London Bridge, this would work melodically too, because the V chord for D♭maj
is A♭7
, and the 3rd of an A♭7
chord is C
--the same as the final melody note. Here's how it would look.
Lyrics: my fair la-dy ... London
Original chords: G7 G7 Cmaj ... Cmaj
New chords: G7 G7 Cmaj A♭7 ... Dbmaj
As long as you can turn the melody into a common/shared tone between the old harmony and the new harmony, the transition will be smooth. Among these options, there is no objectively smoothest transition--it will be a matter of personal taste and familiarity. If you can keep the root the same and change only the quality (e.g., change the final chord from Cmaj
to C7
, with the idea of modulating up to Fmaj
), then the transition will be even subtler. This tends to favor modulating up by fourths as opposed to modulating up by fifths.