Happy Air is a Thai budget airline, founded in 2009. The planes are small but make a good impression and seem well maintained. Happy Air is one of the fastest ways to reach the Mergui, specially if you are coming from Bangkok. Flights depart from Suvarnabhumi Airport, Thailand's main international hub, and arrive in Ranong, the Thai border town.
Happy Air's daily flights take about 1.5 hours and cost something in the area of 2,500 Thai Baht. Check their website for more information.
UPDATE: Happy Air has not been operating their route Bangkok-Ranong during the past months, we recommend to fly with Nok Air.