Anything except cedar. If you use cedar make sure the wood is aired out and sealed so they don't get sick. I prefer pine with a stain/sealer combo because of the cost and strength. I have a raised untreated pine coop that is painted on top but not on the supports and has survived flooding at the supports several times and shows no sign of wear after 2 years. I also have 2 pine coops that have been treated with stain/sealer combo that look like redwood. With a good quality stain you can make pine look like just about any wood.
