Great question. For me, its Delta or Southwest. I like Delta because they fly everywhere, good service, nice planes, no problems. Southwest is great, although they are somewhat limited on their cities (can't fly into smaller cities, and sometimes have to use alternate airports). Open seating is fine, crews are friendly, flights usually ontime, and the BEST is that you can get a credit when your fare goes down and NO change fees. I've given up on US Airways, far more problems than I could ever imagine...