I am asked occasionally why I cram so many different chords in a guitar accompaniment. It isn't a wish to see how many you can contrive, it's that the less chords you use, the better your voice has to be...

A single chord song? I have been known to sing Dylan's Ballad of Hollis Brown with one chord and a single hammer other note. Guitar tuned to double drop D and capo'd up to E to suit my voice, I think.

Of course, using a modal tuning and running arpeggios around the fundamental chord sounds nice and when you think about it, makes most songs single chord.