nice roof top bars that i have not visited yet
http://www.thaivisa.com/forum/topic/785303-rooftop-bars-bangkok/
Zeppelin is okay, although it lacks a bit of atmosphere and the view isn't that great
but it does have a glass walkway with a horrifying drop beneath it which will make any legs seem unsteady after a few drinks and is worth it for that.
Above Eleven on the 33rd floor of the Fraser Suites at Suk 11 is pretty nice but it does get quite busy at the weekend. Probably the best location wise for being near the nightlife, although it depends where your nightlife takes you.
I quite like Speakeasy at Hotel Muse- not the biggest place but the drinks and food are good.
Cloud 47 at Silom is worth a look, not the most comfortable but it's got a weird futuristic style of it's own- not been there in about a year so don't know how busy it gets.
Rooftop restaurant at Muse Thonglor is pretty good as well- view aren't the best but it's right on top of the club so that's the shortest distance between cocktails and nightlife I can think of.