As we know from the fact that many people sing accompanied by
guitar (classical, folk, electric, etc.), the best match for guitar seems to be the human voice, the vocal. Some people compare the human voice to an instrument, which makes sense.
Other instruments that go well with the classic guitar are the
piano, pianica, and harmonica.
The ukulele, banjo, bass, and fiddle types follow, and a
heterogeneous combination is the flute and organ.
Personally, I have played with piano, harmonica, and flute so
If the volume of the accompanying instruments is too big, not
matching the volume of the classical guitar, which has a low volume, the guitar should be replaced by the electric gut guitar with an amplifier and speakers to balance the volume.