I can't recommend an actual company but the general opinion is Koh NangYuan, off Koh Phangan is the best spot in the Gulf. On the Andaman side the islands in Trang province are highly rated, and of course many islands in the Phuket/PhangNga region. Depending on the season I'd start at either Phuket/PP or Koh Phangan/Nang Yuan.