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?

Given a choice between James Taylor and polka, I'd choose polka.

No Erik you're actually quite right. Have a listen to Big Daddy Wilsons' song "If you were mine". Just his vocal and stand up bass. One of my favorite demo tracks.

@hickamore - Speaking of mopey male vocals, how do you feel about Leonard Cohen?

If forced to choose one today it's piano. Tomorrow who knows!