For my pine floating shelves, and pipe shelves, I use locally sourced pine wood. It is very Important to note that this wood will have all of the characteristics of pine wood. Mainly, knots, uneven grain patterns, small cracking, etc. I really feel that this enhances the character of the shelf, but please keep this in mind as you are shopping.
For my heavy duty hardwood shelves, I use either walnut, cherry, or hard maple.