I stayed in Bangkok in January 2010. I decided to spend few nights in Banyan Tree Hotel. I compared Banyan Tree to other hotels including Shangrila, Mariott, Lebua and it thrived.
Banyan Tree has excellent service and excellent food (breakfast included). People are pleased but accept your privacy. It is very luxury and meantime quiet hotel. It has all neccessary ammenities including fast internet connection for free. There is an amazing open bar " vertigo blue moon' on 59th floor - it is worth having few late evening drinks there (mojito is 10usd). Just drink & breathe. One of disadvantages might be location (it takes 5-10 min to get to the river (boat is a fastest way of travelling in Bangkok) but on the other hand you a 5-10 min from a night market Pat Pong that offers a great buys.
Book a prime room 50-55 floor offering amazing view over Bangkok.
there are cheaper hotels in Bangkok but if you want to taste world class luxury book Banyan Tree.