Ko Samui

Bo Phut beach
photo by flickr user g-hat

very popular and developed island in the Gulf of Thailand, sister island of Ko Phangan

Wikitravel says:

In the early 1970s the first backpackers traveling on the back of a coconut boat arrived on Ko Samui. For years after that the island just had a few bungalows and a trickle of travelers. Things started to change in the early 1990s when tourists started arriving in full boats and since then the place grew substantially. Samui is now the second most popular place as an island destination in Thailand (first is Phuket). Ko Samui may not be the country’s most beautiful island but it is still an oasis of natural beauty with its white sandy beaches, dazzling coral, luscious lagoons, picturesque waterfalls, swaying coconut trees and crystal clear water.
Unfortunately, development on Ko Samui is starting to take its toll and the beaches of Chaweng and Lamai are overcrowded in the high season.

Wikipedia says:
Ko Samui island of Surat Thani Province (or Koh Samui, Thai: เกาะสมุย), or often, simply Samui as it is referred to by locals, is an island off the east coast of the Kra Isthmus in Thailand, close to the mainland Surat Thani town. It is Thailand's third largest island, with an area of 228.7 km² and a population of over 50,000 (2008). It is rich with natural resources, white sandy beaches, coral reefs and coconut trees. ... read more