A double bass is the best instrument to pair with vocals

I would prefer acoustic guitar 

Accordion.... 🤣

Acoustic Piano.

you will love this

FYI: both Sheila Jordan and Helen Merrill have recorded albums with just bass for accompaniment.  

and the answer is piano.

As most mentioned so far it's indeed piano

Piano:

DeKay

piano and clas. guitar

mellotron

Also, you're not "trolling" -- more like trawling.

Here is an example of double bass & voice that shows it’s not at all crazy and can be incredible, at least if the bassist is Ron Carter:

@hilde45 

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!

Musica Nuda nicely pairs double bass with a female vocal. 

Beat this :

Sinne Eeg : Staying in Touch

Musicianship and recording are out of this world!

@mike_in_nc  Leonard Cohen was a near-incomparable songwriter with an unforgettable, wrenchingly evocative baritone. Where Taylor wrote nothing, Cohen wrote widely and profoundly; where Cohen's work is endlessly covered by others, Taylor made travesty covers of the work of others, Cohen included -- his "interpretations" invariably clueless, lifeless, dumbed-down, always missing both the point and the mood, and recorded with the dynamics of dishrags. Taylor's voice -- a thin, reedy tenor with no range -- wouldn't have made him soloist in your average metropolitan church choir. You ask me to compare a world-class original solo genius vs. a talentless mooch born into the right social class at the right moment, getting through by licking the boots of the great artists: Mark Knopfler, Joni Mitchell, Carole King, Elvis Costello, even Neil Young once, who allowed him to perform with them, presumably out of charity? His ex-wife Carly Simon had more talent in her toenails than poor Jim on his best day. Who can imagine Taylor writing or performing something equal to "The Way I've Always Heard It Should Be?"

Hick,

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

@hickamore - Thanks for clarifying that. We will disagree about Cohen’s singing forever; but I will look for some covers of his songs to see if anyone else sings them more to my taste. (While no one’s music can be happy all the time (Haydn and Mozart came close), I have found LC’s work dreary and mopey and draggy -- but that’s just me, and I'm not much of judge of pop music.)

@crustycoot   Did you not notice that I was asked for a comparison of Taylor with Leonard Cohen? That is what I supplied. Already I had signified that I "didn't like JT," and would happily have let matters rest there.

@mike_in_nc  I'm confident that you will find, if interested, some Cohen covers more to your taste. A band called Brother Wiley recorded a lively, guitar-heavy cover of "First We Take Manhattan" that sheds much of the gloom and bitterness. I brought it up on Qobuz and you could surely do the same. Even the likes of Haydn and Mozart can be allowed four minutes' rest ;-)   

@hickamore - Thanks; I'll check them out.

Re: Mozart & Haydn, I do like Shostakovitch, too. No one ever accused his music of being happy all the time!

my fave vocal pairing, was slam stewart and major holley both duetting on their respective string basses, on a delightful album "SHUT YO MOUTH" which is notable for major holley's basso profundo singing down low in pitch-unison with his string bass all the way down to low E! what a voice!

Steinway Model D