“What’s the best beach in Thailand?” and “What’s the best island in Thailand?” seem to two of the most common questions on Lonely Planet’s awesome Thorntree Question and Answer forum.
A frequent Thorntree user, who I only know by his username “Tezza,” has created a very useful website to help travelers answer the “Which beach/island?” question, based on his own very extensive explorations of Thailand’s Islands and Beaches.
The index for Tezza’s website is here and is the best place to start.
If you’re planning on visiting Thailand, the most important determinant of which island you choose is likely to be which month you’re traveling in. The east and west coasts of Thailand have different monsoon seasons so some months are best for west coast travel and during other months east coast travel is best- more info.
I also like Tezza’s list (with pics) of the most pristine beaches in Thailand.
Judgments about beaches are very subjective so I recommend you always look up multiple sources of information before deciding which beach/island to visit. Lonely Planet’s Thorntree Thailand branch is a good source of information and a good place to ask questions.