If you're not going with stabilized materials, then you want wood to be a) dry, b) hard enough to withstand normal handling wear, c) attractive enough to use on a knife. With respect to hardness, the test I use is how easily it is for me to put a dent in the wood with my thumbnail. Some maples--silver maple (sold in the lumber section at the big box stores), big leaf maple, etc.--are pretty soft, and knife handles made with them will show dings after some use. Sugar maple is much harder, and so far for me has been more durable. As far as attractiveness, you just have to decide what appeals to you as far as color and grain patterns, and how it fits with the overall design of the knife. Some relatively plain walnut or maple can look nice if mosaic pins or other embellishments are added.