Pine is a fantastic choice for a chair that is going to be somewhat protected from the elements especially on a covered porch. Pine is lightweight, easy to move and has a 'warm feeling' when sitting in it. I reccomend an annual application of a stain or paint to maximize the longevity of your chair. This is by far the most popular choice because of the price.
Cypress is a more expensive than standard white pine has similar workability but contains more natural oils which resist rotting and bug infestation. It has similar weight and feeling as pine, while providing more resistance to rot naturally weathering to a silver gray when untreated.
White oak, not to be confused with the more commonly available Red Oak, is a very durable hardwood when used for exterior projects. It does provide some challenges when building the chairs, dulling blades, breaking drill bits and snapping screws. In the end however, you get a chair that is noticable heavier and provides a more solid and cool feeling seat when compared to pine. This is my second most popular material to use. I still reccomend a regular application of finish (stain or paint) or alternatively you can allow it to turn silver grey naturally.
Pressure treated lumber balances value and durability. Often the pressure treated boards I use have few knots and fairly clear grain. It has very good resistance to rot and bug infestation.
Pressure treated lumber does contain chemicals which some customers find unfavorable.