Mickey's Warning (aka Blue Bleezin Blind Drunk).

Anything from Adam McNaughtan, since that's how he always does them - e.g. Cholesterol, Remember Thomas Muir of Huntershill.

Most of Matt McGinn, since he also sang unaccompanied most of the time - On the Beach at Portobello, say.

I can't actually think of many songs that wouldn't be improved by leaving the guitar out.