I find that the most consonant 7th chord in all of music is the minor 7th. But why is this? Well here is how I rank the common 7th chords from most dissonant to least dissonant(theoretically there are 12 7th chords you can get from a single root but the ones with an augmented triad base are very rare):
- Diminished 7th
- Half diminished 7th
- Minor major 7th
- Major 7th
- Dominant 7th
- Minor 7th
And here are some audio examples of different 7th chords.
Here is what the diminished 7th sounds like:
I often hear these 2 chords together like this:
Half Diminished 7th goes to Diminished 7th on the same root, which then resolves to tonic.
Now, here is the Minor major 7th and its resolution:
I don't know if you would order these 7th chords in the same order in terms of dissonance but I'm pretty sure that everyone would agree that the minor 7th is the most consonant of all these 7th chords. Here is my thinking as to why it sounds so consonant. It has to do with triad overlap.
7th Chord Types and their upper and lower triads(upper triad being third, fifth, and seventh):
Diminished 7th: Diminished triad base with Diminished upper triad
Half Diminished 7th: Diminished triad base with Minor upper triad
Minor Major 7th: Minor triad base with Augmented upper triad
Major 7th: Major triad base with Minor upper triad
Dominant 7th: Major triad base with Diminished upper triad
Minor 7th: Minor triad base with Major upper triad
Now, according to this, the Major 7th and Minor 7th should be equally consonant. But I don't hear it as such. This is where chord relations come in. If you were to take a major 7th apart into its 2 constituent triads, you would have I and iii. In the key of C, that is C major and E minor. These 2 chords are closely related, but a direct modulation does not sound all that good between these keys, or at least not as good as a direct modulation to the relative minor. In contrast, with the Minor 7th you have i and III. If C is the tonic, that is C minor and Eb major. These are not only closely related keys but they are also relative keys. This makes the Minor 7th the most consonant 7th chord, at least, I think that is what makes the Minor 7th the most consonant 7th chord.
Is this triad overlap between relative keys the reason why the Minor 7th sounds much more consonant than the other 7th chords?