Koh Samui is probably the best known and developed. A totally tropical island with three spectacular beaches and many other small coves. Flying time from Bangkok is 65 minutes or from KL 95 minutes.
Koh Phangan is a 30-minute speedboat ride from Samui, much quieter options with some great beach options, real hide and chic retreats.
Koh Tao is wonderfully tropical, 2 hours by ferry from Samui – even less developed with a rather hippy vibe and just a couple of rather funky chilled out hotels.
Khanom is located on the mainland, facing Samui, and you fly into Surat Thani (75 minutes from Bangkok) and then a one-hour road transfer.
All are generally at their best March to September, although being lush and very tropical rain can fall throughout the year. December to February can see also some beach erosion and higher sea swells.
Samui – Good, highly innovative hotels and some excellent dining options. The majority of the development has occurred in the north-west area of the island, closest to the airport and the more expansive beaches. Chaweng is the most popular and crowded beach, with many restaurant and bar options. Mae Nam and BoPhut are equally good beaches but with fewer crowds and a more family/romance focus. On the west and south coasts, it is still wonderfully real and authentic although the beaches are less attractive. It is easy to explore, plus you can easily visit some of the offshore islands.
Phangan, Koh Tao and Khanom are much less developed, it is like going back 30 years in time, with just a handful of hotels, and offers a true sense of adventure. The beaches at Khanom are incredible, some of the very best in the region and totally unspoilt, and the scenic contrasts amazing. Koh Tao rejoices in its laid back almost bohemian style, its a bit Thai wild west in places with local versions of Lemmy from Motorhead in abundance. Koh Phangan, despite being one of the largest islands is still wonderfully tropical and lush with some superb beaches and just a handful of international styled hotels. It is known for its rather wild and hedonistic monthly Full Moon parties but that is just on one beach and can be easily avoided.
Samui seems to have focused on building hotels without a lot of thought on the island infrastructure, it can show at times. Lively Chaweng is an acquired taste but it has restaurants and bars galore. It is a small island so hotels on the popular north coast, within 15 minutes of the airport, might see/hear aircraft.
Phangan, Koh Tao and Khanom will appeal to the curious travellers looking to experience something new and fresh. They appeal particularly to those who have visited Thailand as backpackers in their student days, loved it but can now afford something rather more luxurious. Or those who really enjoy Thailand but feel the resorts of Phuket and Samui are now just a little too busy.