The best woods for banjo building are hardwoods. Maple, walnut, and cherry are some of the best woods for banjo building. Some builders choose to use apple, mahogany, sycamore, ash, alder, poplar, hickory, and on and on. These woods are all acceptable. I usually prefer to make the fingerboard and accent pieces like the peghead overlay, rim cap, and the heel cap from material that contrasts with the wood that the rest of the instrument is made from.
The main thing in the choice of banjo wood is that it be strong but not too heavy. Pine, cedar, and spruce are all too light, and probably not strong enough. Oak and locust woods are a bit on the heavy side for my taste, although they are quite strong. I have seen oak used for tack head banjo rims and for gourd banjo necks. There are varying qualities of wood in all species and that is part of what makes wood so fun, every piece is different! I like to use figured woods for their beauty and challenge. To read more about wood, check out Understanding Wood by R. Bruce Hoadley.