I agree with the above statement that the instructor is the most important thing.
However for the sake of having fun and enjoying the whole experience, try and learn somewhere that will have good vis and is in season, which should ensure better water conditions.
Therefore I believe that in Thailand Nov-March try Krabi, Phi Phi, Phuket, Trang, or Koh Lanta, and between April-Oct try Koh Tao, Pattaya, or Samui.
But if you are going between April - Oct I would actually recommend going down to Malaysia to dive Pulau Tioman, Pulau Redang, Pulau Perhenttian Besar/Kecil, or Pulau Aur, as in my opinion the diving on the Eastern side of Peninsular Malaysia is much better than Diving in the Gulf of Thailand (East Thailand).