I lived in Thailand for 2 years, but would say it depends on what you're looking for:
serenity: Ko Chang, Ko Phi Phi, Krabi
party scene: Ko Tao, or Phuket
something in between: Ko Samui and some beaches on Phuket
My personal two favourites for the scenery are Ko Phi Phi and Krabi.
Keep in mind this was several years ago so things could have changed.