If you go to a real Lumber-Yard they will be able to help you better.
I would suggest redwood or cedar though they might have another suggestion based on your climate. They can have the lumber milled to any dimensions you need, Just tell them what you want (i.e. square corners, rough sawn, smooth with round corners, etc.)
Pressure treated lumber is meant for contact with concrete or earth and is not considered the best choice for above ground use.