if you want a quiet Island, chose either Koh Phi Phi or Koh Pangan. Do take in account that Full Moon parties take place at Koh Panghan onve a month, so if that somes out during your time there - chose Koh Phi Phi (the movie "the beach" with Leo DiCaprio was filmed there...)
these 2 choices are small quiet islands, absolutely beautiful, with smaller hotels etc. as opposed to the bigger Thai Islands that are busier and touristy-er.
enjoy!!