Depends what time you arrive in BKK if before 6pm then probably Bell Travel service
http://www.belltravelservice.com/routes_rates.php# is your best bet. Later than that I would book a taxi through Mr T via email
mr_ttaxi@hotmail.com or if you feel the urge to get to Pattaya as quickly as possible then book a taxi even before 6.00pm.
I'll be here in Pattaya and can show you the delights of Soi 6 either pm me or call me my number is 0845674080