Below is an email I sent to a group of music nerd friends, including The Doctor of Rock.
Here are the two main qualities that make a good chord:
1. At least one note from outside the song's key. This includes chords like II, VI, VII in one direction and bIII, bVI, bVII (and I suppose iv) in the other.
2. Somewhat unexpected given the preceding context. So the bVII in a song that just goes back and forth between I and bVII is not as good as the bVII in Flashdance, which is pretty close to the ideal chord. And the III in a song that's obviously minor is not as good as the III in the chorus of Glory of Love.
Sometimes an entire song section can be chock full of good chords, to the point where it's hard for me to assign "good chord" credit to individual chords. Like the prechorus and chorus of Borderline. This is also a case where the sequencing of the chords helps a lot; the chorus *starts and ends on the V*.
Another thing I like is when II-like chords and bVII-like chords appear in close proximity. Like the bVII and III in the prechorus to Starlight. More generally I'd say the weakest use of II is in a II - V, and the weakest use of III is in a III - vi. Not weak, just weaker than other uses of II and III.
Sometimes a song has a chord that I think I can improve on. Are You Happy Now by Michelle Branch is a pretty good song, but the ii in the chorus should *clearly* be a II. Dream a Little Dream has a bVI that should *clearly* be a iiø7.
