Frontier might not be your first choice of airlines, but when seeking a budget-friendly plane ticket it's usually the lowest there is. The airline often puts out flash sales on domestic flights for as low as $20, but the overall flight experience may not be worth it. More than 35 percent of Frontier's customers rated their experience with the airline as "terrible" on TripAdvisor. The various reports of long delays and cancelled flights are what put it so low on this list.