All of those places can be wonderful if you choose the right place to stay. Given your flight itinerary, I would probably choose Phuket and do a one day speedboat excursion to the Phi Phi islands. It is also possible to get to Krabi by car from HKT, but it is a bit of a long drive.

I would make the decision based upon which specific resort I wanted to stay in. If you stay in a desirable location, either Phuket or Krabi will be perfect.