Cottonwood and some other very soft hardwoods can be avoided. Likewise
anything without much color or grain. Other than that, I would try about
anything. Even more plain woods like maple are good with natural edges or
with the interesting grain of crotch wood. Go for interesting colored
woods.

I have done apple, walnut, maple, sycamore, cherry, plum, maple burl,
ponderosa pine, aspen, black locust, and many more. Tony Manella recently
noted that he had turned about 60 different types of wood.

Derek