Beautiful Bay that you can reach by boat from Phuket but ask in a few places before choosing a 'driver' as prices might differ... Totally worth to visit! Phang Nga reminds me of Halong Bay in Vietnam.
Phi Phi have the most beautiful shade of water around and the surroundings are just stunning - but underneath there's a lot of trash. Locals don't care about collecting rubbish and saving the environment too much unfortunately. There are many local restaurants and places to visit which is nice.
Phuket is a beautiful island, a bit touristic which often means expensive too but it's worth to come and explore. There are many tours in offer, food is as great as everywhere in Thailand, nature is beautiful. There are so many beaches it's hard to explore all of them.