Thailand forum

There are some nice areas on Samui. Guess it depends on what you want. The north shore has some really nice, quiet areas, and some lovely stretches of beach.

If you want isolation and silence, this is not the island. But it is an easy place to get to and around on.

As somtan suggests, the Samui and neighbours tend to have the best weather in September. All other island areas are normally in wet season.
Samui is pretty and has the best selection of midrange and better places. It is not so good for budget travellers these days - for them the neighbouring islands of Ko Phangan and Ko Tao are the go. These do have some very nice more expensive places, but not the huge selection that is on Samui.
Samui also has the advantage of being able to fly directly onto it. The only other Thai island you can do this is Phuket, which usually is considerably wetter in September.
Samui isn't great for snorkelling off the beach - in the area Ko Tao is the best for this, as it is for dive operations.
Samui is the closest and has the most operators going to the fabulous Ang Thong Marine National Park islands which are the best daytrips in the area.