There are so many different species of wood available at your lumber yard nowadays, that it may be difficult to know where to start. Your choice will depend on a number of factors, including cost, color, grain and availability. If you are unsure as to what particular wood you should use for a particular project, talk to the people at the lumber yard as they will be able to guide you very well.
All wood is not created equally. Wood with good working properties such as basswood, or alder, may not have the strength or durability for your project. At the same time, other woods such as cherry, or walnut that have nice finishing qualities may not be the color of choice for the project.
Choosing the best kind of wood for your project might be difficult given that you have many options to consider such as cost, color, grain pattern, hardwood/softwood, expansion/contraction with relative humidity, kind of finish to be applied, and a few more. But choice is a good thing and there are many species of tree that will probably fit your project's needs.
Here is a list of common woods and their durabilities to the exterior environment.
Here is a list of common woods in order of density from (least to most).