My own preference is to try to avoid or minimize the use of endangerd tropical hardwoods and other exotic materials, in favor of those that are sensibly and sustainably harvested. Ideally, I like to find the tree, log or plank of wood myself and know where it came from and who cut it. Often this simply isn't possible, or the customer has specific needs or desires that can't be met within the limits of this approach.
Some woods, like ebony for finger boards, serve a function that is hard to replace. Still, there are a number of wonderful domestic hardwoods that make great guitars, and I've found a few locally in California and on the west coast that I've become very fond of using. I have on hand beautiful, figured Black Walnut, flamed Big Leaf Maple, and some amazing local California Bay Laurel (known in Oregon as "Myrtle Wood"), as well as odds and ends of other species.
For tops, I've used a lot of local Redwood, and am currently using less of it. I find it a bit challenging to work with, and it simply isn't as strong as most of the other top woods, so for a multi-string harp guitar it wouldn't be my first choice (I especially like Sitka spruce for those). But it can make a lovely sounding and looking lower tension instrument. Of course, I have a stock of some of the more common topwoods, as well: Western Red Cedar, Sitka Spruce and Engelmann Spruce. Any of these can make a great guitar, and offer a range of tonal and aesthetic qualities.
Beginning in 2016, I've been moving towards more banjo-esque construction. Wood choices are still the same, but for tops (in banjo jargon the top is called the "head), I vastly prefer a natural animal skin as opposed to mylar plastic. Mostly I'm using fairly thin goat-skin; it has interesting visual qualities, and a lovely, warm-but-bright sound. I like to color it with paints or "kakishibu" (persimmon dye).
ALIRECEL: Industrial-quality papier mache
Off and on for over 20 years, I've experimented with using techniques I learned for making theatrical face-masks, to make back/side shells for stringed instruments. The material I use is recycled paper grocery bags, saturated in aliphatic resin glue. This material is pressed or layered into a plaster negative mold made from a clay sculpture. The name "ALIRECEL" is an acronym for Aliphatic Resin Cellulose Laminate; one of my students thought I needed a more impressive name than "paper mache". Whatever....
In any case, it makes a very hard, thin, resonant shell when done correctly, and can be cut, glued and painted. I've found it to be challenging to build a guitar-sized instrument with it, but it's quite manageable for a banjo back/shell, and really works well for that. A variety of paints can be used to decorate it; because of my sensitivity to acrylics, I've been using artist's watercolors with a protective over-coating of home-made violin spirit-varnish.
Many years ago I had to finally give up using nitrocellulose lacquer. I'd developed such a sensitivity to it's toxicity that I can't even open a can of it in the shop now. It's dreadful stuff, and I no longer want to promote it. For a number of years I used one or another of the "waterborne polymer" finishes, which generally seem to be some combination of acrylic and urethane resins. Although they apparently contain less volatile organic compounds than traditional lacquer, and are therefore considered to be safer for the environment, this type of finish still contains solvents that are not good to breathe. Because of my chemical sensitivities, I started to get sick using this material, though not as much as with lacquer. Investing in an elaborate powered-air-respirator initially helped with the problem, but I began to get sick simply from an instrument "gassing off", even after the finish had cured for a week or so. In the summer of 2005 I finally decided to learn how to French Polish, using grain alcohol and Kusmi seedlac, with nothing else but a tiny bit of oil in the later stages of application and polishing. Although I'm slightly bothered by the alcohol fumes, I've been impressed that there is no detectable gassing off after 12 hours or so, and I can even do some of the application with no respirator. The finish is beautiful, earthy (looks much more like a "vintage" instrument finish), and seems harder than the waterborne stuff I'd been using; I now finish all my tops with this. For backs and sides I continue to use an oil-based finish. The product I currently use is "Tried and True Varnish-Oil". This is a 19th century cabinetry finish made from linseed oil and pine resin. I put on enough to protect the wood and bring out the beauty of the grain, but I don't try to build it up to a glossy coating. It has a warm, satin sheen and feels wonderful to touch.