I think Bangkok Airways are by the far the best domestic airline in Thailand, possibly even in the world. Where else can you get free lounge access for all passengers, free meal on board, free luggage, free seat selection and more.
This flight from Chiang Mai to Bangkok Suvarnabhumi on an Airbus A319 only cost me £30 but included all above. Watch my full flight including, check in, lounge, boarding, take off, service on board and landing.