If you are diving from Koh Phangan you should go with The Dive Inn. They have the best, nicest, most professional instructors, a comfortable boat and good, clean diving gear.
They really look after you on your trip, and know all the most interesting things to show you.
The dive shop is right next to the pier, so it's quick to get on and off the boat, and there is a good cafe next door, so you can get a decent coffee before and after your dives.
Highly recommended. You will have great dives.