Not been to the Banyan Tree, but was around the Laguna complex earlier this year, which the Banyan Tree is part of. The Banyan is regarded as the best one on the complex. Laguna felt more like Florida than Thailand, very manicured with a large lagoon and Disney like boats plying from hotel to hotel. Bang Tao Bay beach is good, with a selection of restaurants on it.

Weatherwise Phuket best time would be Dec to March/Early April. These are the times I have been and I've never seen any rain (or even a cloudy day).

Plenty of five star establishments in Bangkok...there is a Banyan Tree for example. Many people rave on the Peninsula which is well placed on the river. The Oriental again on the river is world famous.