Any place that's warm is a good place to learn! Ive been to Thailand many times. Phuket is generally considered to have some of the better diving in Thailand. Most of the better dive sites are going to be a good distance out. Phuket has a lot of dive shops!
You might want to also look at Koh Lanta, which is a little further south. I was there last year. It's much less developed. That said, you'll still find all the tourist facilities you'll need. The diving is comparable or maybe even a little better. Again, the sites are fairly far out.
Koh Tao is on the gulf side, and the viz isnt good. On the plus side, the dive sites are much closer and is much less developed than Phuket. It can be a little challenging to get to.
One other place you might want to look at is Koh Chang. It's starting to develop and might be a good fit for you. While Thailand is one of my favorite places to visit, if diving is your main purpose for a visit, you might want to look at the Philippines as well. While not as interesting topside, the diving is better.