Since I got certified in 2001, I've been diving in the following places in order of dive trip: Cheju Island Korea (Open Water), Boracay Philippines (Advanced Open Water), Phuket Thailand, Koh Tao Thailand, Saipan CNMI, Puerto Gallera Philippines, Palawan Philippines, Palau, Gili Trawangan Indonesia and Bohol Philippines. I have a real interest in doing a liveaboard on the Great Barrier Reef in Australia one day and another trip to Palau before I leave this area. Eventually I'd like to dive the Red Sea as well, which would allow me to see the Pyramids and the Sphinx and all that stuff everyone should see.
Cheju Island is between Korea and Japan. It is the Korean honeymoon spot. I'd never even been snorkeling at this point, let alone diving, and then I did my first dive here. I thought it was amazing, but anything would have been amazing on dive one. The first dive is like the first sexual experience: awkward, nerve wracking, amazing and very short with that little voice saying, "everyone should know about this, wow." I doubt this is a must see spot for serious divers, but is good if you are in Korea.
Boracay, Philippines. This place has a great beach, good restaurants and a cool atmosphere overall. I just went for like the fourth time a few weeks ago. The diving here is fairly lame, but it is a great place to learn to dive because the water is warm, clear and there is no current. This is not a place for experienced divers. I did a drift dive that was cool, and there is a deep dive called Yap Bak or Yap Yak or something like that, which is ok.
Phuket is a bit of a tourist hell along the main tourist areas with the relentless commercial assault, but there is really no place like Thailand. There are lots of cool day trips to take to caves, elephant rides and ancient cave drawings. It is legitimately gorgeous here, the food is great and the diving is spectacular. The diving is also not cheap for Thailand, or anywhere else, as you have to go out pretty far to get to the good spots, but there is tons to see here. I dove with Sunrise Divers and recommend them.
Koh Tao has a great reputation for diving, and I thought it was pretty good, but not great. You get there by ferry from the Koh Samui area, and it is a small island with a good vibe, good food and good, but not great diving. The whole focus is on diving here, unlike nearby Koh Phangan, the stoner "full moon party" island. The diving is cheap, though. $20 bucks with gear when I was there. It doesn't have the commercial assault so evident elsewhere and the bungalows are like $5 per night. There is no power during the day for the most part, but everyone is out diving anyway.
Saipan, CNMI. We are lucky to have such great visibility here, but it certainly appears to me that the coral is getting bleached out. I watched the Mike Tripp video and this was especially evident. The Grotto is still a staggering dive, and I like Naftan and Ice Cream as well. I'd like to do Bonzai when the time is right, but I've never done so thus far. I wish I could do more boat dives here without breaking the bank. I only spent more money on diving in Palau than Saipan -- at least when I came here as a tourist. It's good here, but not amazing. I'm eager to dive Rota as well, but have not thus far.