I looked at Koa Tao and I'm planning to do the PADI elearning then the dive training elsewhere. What I'm looking for is the best value for the dive training. Anyone have experience with Koh Tao?
Koh Tao is definitely among the top few places if you want to go all or much of the way on the "zero to hero" road. Utila is another good option, but I've got less experience with it.
It's great if you plan on staying long, doing OW, AOW, a few specialties, EFR+Rescue, maybe even more. The rent's cheap, the food's cheap, it's great weather year round, and since it's an instructor factory, there's so many around that you can get a student-to-teacher ratio of just 2:1 or even 1:1, except in peak season. Safety standards are among the strictest, and the shops are well-equipped, with training pools and a lot of kit.
Instruction quality is generally good, since the instructors still remember their IDC, and most major agencies are represented on Koh Tao, including RAID. If you want the best training, I'd strongly recommend going with RAID - you'll spend a little more, but the classes take longer, cover very important buoyancy and trim skills, and many instructors will teach you till you actually feel at home in the water. You can get great instructors with any agency, but tech-rooted agencies are much more reliable at that.
Diving on KT isn't the best, though, due to the flow of tourists murking up the water. All of the dive sites are quite safe - shallow bottom, slow currents, and lots of boats around. But it's enough for a beginner, and there's enough sites not to get bored for the first couple months. At some point you'll definitely want to try out other styles of diving that you can't get there, though. Cozumel and the Philippines have considerably more options.
edit: Given your background and plans to dive all over the world, definitely go with a tech-rooted agency over PADI, or otherwise choose your instructors thoroughly. You'll want to get a tec instructor that will teach with wings and make sure you can get your trim and buoyancy right every time. That will matter a lot when diving in different waters. And with your background you'll probably learn more efficiently from technical divers as instructors anyway; I've experienced how it can be a barrier to learning when the teacher is younger and less capable of taking things seriously than the student.