Depending on your priority, for best diving, the Similans/Richeliue/Surin are definitely the place to go, also not to miss the south Andaman Sea, Phi Phi, Tarutao, Krabi etc.
However for Similans, you really need to do live aboard as you will need to spend quite a lot of time commuting for day trip if you stay in Phuket or Koh Lak. Speedboat is not all that comfortable except on an absolutely calm day.
Koh Tao diving is so so but it gives you a nice relaxing environment where you get to relax on a small cozy island and most dive sites except Sail Rock is nearby.
However for Similans, you really need to do live aboard as you will need to spend quite a lot of time commuting for day trip if you stay in Phuket or Koh Lak. Speedboat is not all that comfortable except on an absolutely calm day.
Koh Tao diving is so so but it gives you a nice relaxing environment where you get to relax on a small cozy island and most dive sites except Sail Rock is nearby.