Phuket , don't go near it unless your willing to pay 3 times the odds for everything , very overcrowded , think costa del highrise. though it is a good transit point for getting to Ko Phi Phi don ( Large Island beside where "The Beach " was shot.
Krabi should be investigated big time , beaches out of this world , imagine your dream tropical beach, then multiply by 10!! Ton sai and raili ( sp?) are two def worth checking out
Ko Samui is fine if you want the big glitzy resort type holiday , but look for somewhere away from Chewang beach, crazy place where main drag has more fish and chip shops/pizza hut than traditional thai glass noodle stalls.
Bangkok is hard to take at first but having been in and out Soi Sukumvit is the place to stay
www.asiarooms.com never let me down, sort by area.
On most of the streets off Soi Sukumvit there are travel agents that do great day trips out of Bangkok, Bridge over River Kwai trip is well worth doing , do not book in hotel , rip off.
Chaing Mai again as previous poster said , well worth a visit , its in my top three favorite places in Thailand , fly from BKK , 1 hour and should cost no more than €60 odd return , then fly from C.Mai-BKK-Samui or Phuket to go to Krabi or Phi Phi.
Lots of trips on offer in C.Mai , scenery stunning , i found it way cheaper the further North you go in Thailand whilst down south esp on islands its more expensive.
Enjoy your hols.