I did my first ever diving early this year with Lotus Dive ->
Lotus Dive - Discover the Andaman Sea - Koh Lipe, Thailand
I'm not sure I can really compare them to any other shop on the island, but they got me hooked and seemed very professional and highly friendly. I actaully went and interviewed all the shops on the island, and they all seemed very switched on, I picked Lotus simply because they seemed the most accomodating.
I stayed part time at Serendipity -
Koh Lipe Resort Thailand - Serendipity Resort Koh Lipe - brilliant but cheap, and you must be OK with lots of stairs as it is literally on the side of a hill. I did find the manager a little bit hopeless, but the accom and food was great.
Also stayed at Idyllic -
Idyllic Concept Resort ( Koh Lipe )- An idialized concept resort on Koh Lipe - more of a luxury resort, but with the friendliest staff you will ever meet. If you have young children, they will basically kidnap them and take them off your hands. There's also decent snorkelling right out the front of the resort. Both are virtually next to each other.
There's also a lot of cheaper bungalow style accomodation.
Koh Adang is a national park, and there's only one resort on there - owned by the chief of police. I don't think you will win friends by staying there. It didn't seem that special when I saw it - we stopped off there one day for lunch between dives.
Now on the way back to the mainland on the high-speed ferry, we stopped off at a number of smaller islands that looked very very interesting. If I went back i'd do some more research to find out what these were and if any had any diving.