What is the most common type of wood used in making quality musical instruments?

Spruce according to Marty Rainey the man who does 'Homestead Rescue'.He waxed lyrical about spruce being the best.

I'm not sure this can really be answered, but...

Violins, violas, and other members of the orchestral string family, are typically made of spruce and maple, with ebony fingerboards.

Acoustic guitars are often made of the same materials (Electric guitars can be made of almost any material)

Ebony, maple, and grenadilla are used for most woodwinds, except flutes and saxophones which are generally brass, which is often plated.

Most brass instruments are made of, well, brass.