depends on what you're using it for really. certain materials have different properties. the wands i've made have all come from a lilac bush in my back yard i always have felt connected with. if you don't feel a connection with a particular tree, and just want a good want, oak is a good choice. acorn is a second i hear a lot. third is probably ash, research the woods, figure out what you want the wand to do for you.
it isn't limited to woods mind you, people can add stones to them, or make them out of metals. myself i like wood, like making my own wand, unless one calls to me in a store, i don't see myself buying a metal one.