stevenl
Contributor
Take into account that Phi Phi only has gotten louder the last few years, so no, really not quiet, in that respect Phuket has more to offer.However, it was still not my definition of "quiet," so if I was going to go to Thailand again, I might head down to Lanta for a while.
If you really want quiet, besides Koh lanta you can also consider Koh Lipe.