A double bass is the best instrument to pair with vocals

The Jennifer Warnes track “Ballad of a Runaway Horse” is used by John Hunter to tune speaker placement in his Masters program, featuring voice and acoustic bass. 
But for a more universal answer, it would be piano, as in lieder. Schubert and Mahler are the masters of the artform.  I also adore guitar and voice, whether blues or singer songwriter.  James Taylor anyone?

Hick,

So you don’t like JT?  Is that your point?  Others might not agree…I hope you can accept that.