I would go to Sky Bar (but it's outside). I would not bother with the restaurant(s) at the top of State Tower, where Sky Bar is located. The bar is the best of the best, IMO. Drinks are very expensive, so buy one and sip it ever to slowly, to make it last. The view is awesome.

You might also like the Long Table, Sukhumvit soi 16(?). Not as high up, but nice view, and there is both inside and outside seating. We made a reservation for dinner. If you want to sit in the bar and eat, you might not have to book ahead. Not sure about that.

Red Sky is another option. It is the outdoor (not sure if they also have inside) restaurant/bar on top of the Centara Grande Hotel, located at Central World mall. You can see the big colorful arches at this place from a long distance away. Very easy to spot. I forget how many floors up, but it's definitely rooftop.

How much for a drink Simpson?

I agree sky bar at the top of the state towers is a must. It's expensive. At a guess about £7 for a gin and tonic but worth it for the view or have a soft drink. Make sure when/if you go they take you out to the bar area NOT the side seating area, it's nice but nowhere near as impressive.
Closed in shoes for men must be worn also if you are going with a man.

Been to Bankok many times and we always go here, far nicer than the bar at the top of the Banyan tree (imho)

Thanks for the great tips! We thought about 'Three Sixty' at the Millenium Hilton Hotel. Looks like it has some amazing views but is inside also. Have any of you been there?

Yes I have been its great but for me the Sky bar is the one!! If you have time for both ( and are on that side of the river) go!
Haven't been to the other 2 places Carol mentioned but keep meaning to! Maybe when I am briefly in BKK in September.

360 is fantastic, but not high enough in comparison to sky bar and banyon tree (which i prefer).. there is good food at the hilton too.

doesn't the sofitel on silom or sathorn have an enclosed top floor bar??

360 is great with expansive awesome views of Bangkok but it is indoors and located on the Thonburi side of Bangkok. See here: http://www.flickr.com/photos/asiacamera/3506285430/

Silom area has an indoor top floor bar (albeit not very high) at D'Sens French Restaurant at the Dusit Thani Hotel. See here http://www.flickr.com/photos/masamune/5775746754/

While Sathorn has the excellent Moon Bar and Vertigo Restaurant atop of the Banyan Tree Hotel. See here: http://www.flickr.com/photos/visbeek/3941690325/

I personally prefer Sirocco at State Tower with its Sky Bar and Distil venues due to its dramatic staircase (WOW factor) and proximity to the river. http://www.flickr.com/photos/paulokawai/4589325608/

For complete list of Bangkok rooftop venues, look here: http://milepoint.com/forums/threads/bangkok-rooftop-dining-experience-with-a-view.1461/

Either place, get there early enough to watch the sunset and take a good camera. Make sure to turn your flash off as others (including restaurant management) will mind!

Have fun!

Thought I'd update this: Last night I attended the opening party for L'Appart, the rooftop restaurant and bar of the new Sofitel Sukhumvit. It's on the 32nd floor, and the view from the outdoor balcony is really amazing. I was a bit surprised as, from the location, I didn't think there would be much, but looking up Sukhumvit towards Siam there are a lot of interesting buildings, all lit up at night. The interior is rather home-like. There's an 'open kitchen' room where you can dine and watch the food being made, a room that's more like a living room and even a library.

The food for the party was mostly bread and cheese and champagne, with chocolate covered strawberries for dessert. It was all good (there was sushi too, but I couldn't touch it). No idea what the pricing is, but it's a fancy restaurant in a five-star hotel, so it's not going to be cheap.

Sounds great! Thanks for that report. I'll have to check it out. I really wanted to stay at that Sofitel, but have booked Four Points instead. Maybe I'll re-think?? Anyway, I'll have a look at the L'Appart. I'm back in the city July 19 for 5 days.

Is there a dress code?

Dress code? I don't think so. Tank tops and flip flops might be inappropriate, but I don't think they require a tie or anything like that.

I was thinking about the likes of Sky Bar when I asked that. I don't wear tank tops, but I do wear my FIT Flops shoes a lot. At Sky Bar you must be in smart casual to get in. Just wondering if that also applies at L'Appart. I don't wear FIT Flops to Sky Bar.

I don't think it's as snooty as Sky Bar (local owners often seem to out-snoot international ones) but I suspect you'll feel very out of place there if you don't dress up *a little*.

My own pictures of the place are up on my blog:
http://chiengfa.com/2012/06/25/lappart-bangkok-sofitel-suk/

Wow, nice view!! Thanks for the info. Sky Bar is certainly more dramatic looking. Hope to get to L'Appart in a few weeks.