If the natural look of wood is a priority, use cedar. The heartwood of the tree (the deeper colored red part, not the white sap part) is rot resistant.
Cedar wood produces lengths of timber with straight grain, which makes it easy to cut, saw and nail. A cedar timber is highly resistant to splitting and its heartwood has natural decay resistance. The low density of cedar gives a superior insulation value. It is light weight, easy to work with, easy to finish and possesses outstanding dimensional stability.