Pricey, but well worth it
While not as exquisite as other Banyan Trees in resort locales, the Banyan Tree Bangkok is nonetheless a great choice.
Staying at the Banyan Tree Club level not only gives you a decent size suite with great amenities, but will also score you access to the Club Lounge with free drinks, as well as breakfast at the more exclusive Saffron on the 52nd floor with great views - think of it as the indoor version of the famed Vertigo restaurant on the rooftop.
The Banyan Tree Spa needs no further introduction.
The only downside is that the location is quite a walk away from MRT's Lumphini station and is not reachable from BTS - if you factor in the Bangkok sun - so the only feasible mode of transport is car / taxi, and if you know anything about traffic jams in Bangkok, car / taxi often times is not a desirable mode of transport.