I went to Octave at the Marriott Sukhumvit for the first time this week and enjoyed it quite a bit. Great view, very good service, and from 5-7 p.m. the Happy Hour is 2 for 1 on their "Signature" cocktails (which are all 350-375 thb).

You take the elevator to 45th floor then switch to another elevator to the 48th floor then walk up a flight of stairs to the rooftop bar.