I would hardly class the Lufthansa lounge in BKK as business class. It has a good shower and with a gold senator card you can use it regardless of airline flown.

The new Thai biz lounge is great and on 2 floors. The free massage has stopped but it is stil fantastic.

Bangkok airways are great and Koh Samui airport is great fun. Do try to go to Koh Tao if you have time it is paradise.