I'd honestly say you can't go wrong with Frontier. I'm a Summit level and I don't even live in Colorado. My experience has been very few delays, new aircraft, TV and Stretch seats available for free, or a nominal charge (flat fee, not distance based like UA), really low fares. Plus, if you purchase tickets on the Frontier website you have the option of Classic Plus fares which give you 150% of actual miles flown, or Classic fare which gives you 125%. Downside is that many of the ground employees don't seem to know what they're doing, but they never appear to be rude to passengers, and they usually give the passsenger the benefit of the doubt if they can't figure it out.