UA's BKK flight is now in the summer schedule mode, I think. It arrives just past 9 p.m. (around 11 p.m. in winter). Taking a taxi would be easiest. Just from outside the airport. You can take an airport bus but you need to get off somewhere in downtown and should get a taxi from there. You can walk up to the Hilton from Sukhumvit by taking an A3 (I think) airport bus and getting off at the cross with Wireless Road but that is a bit long way to walk.