I wouldn't be diving if it wasn't for Cozumel, and it's the only place I've dove repeatedly. I am going in 2 weeks, and I that will be the 6th time I have dove there. (1st time 2 dives getting certified Scuba Dive, once 1 dive off a cruise ship, and 3 times for a week or more). I have also dove on Koh Tao, Koh Phangan, Amed, Nusa Penida, Ambergris Caye, and Dahab.
The things I love about Cozumel are the reefs, the large marine life (moray eels, turtles, nurse sharks, sting rays, and eagle rays, and the one hammerhead I saw my last trip), how different the dives can be, the wild ride you sometimes get with a drift dive, seeing the Cathedral, that the dives are all boat dives, and that the dives are fairly close (no 1-2 hours to the dive.) The two dive shops I mainly used have been excellent. You also have the added benefit of being able to either take the ferry across to Playa Del Carmen for the day, or staying there and diving cenotes, which is amazing. I love caves, and I love diving, so this is both of my loves put together! I've done 11 so far, and definately want to add more. The down side is that it's become very difficult to get there from where I live-even if I fly into Cancun. There are no direct flights to Cozumel from my local airport, and even from JFK, Newark or Boston-there still aren't any flights. And the connections are now less than 1 hour, and there is no later flight if you miss your flight. I am having to drive to Boston to get a nonstop to Cancun, and then get my way down to Playa by bus (I travel alone), and then take the ferry across. But the island atmosphere, the food, the people, and the hotels are worth it. It is so much nicer than Playa. I will keep coming back, especially since I get to dive with the best divemaster I've ever experienced (who got me diving!).
My opinion-I had a very bad experience with the dive shop I used in Bali, and that totally spoiled my experience there. The diving was top notch, but everything else was awful. I liked Belize, but I don't think it is better than Cozumel-although it has way more turtles and nurse sharks. Thailand-I had a few great dives, some OK dives, and some awful dives. Also, shore dives are difficult for me, as I broke my right leg badly, and so my right ankle is dicey. ( I am a 67 year old woman, so even just walking with all the dive gear is no mean feat.) The water clarity can be really awful. I saw some really cool things there. Dahab was great, except it was all shore dives, and those are difficult for me. So, from my diving experience, Cozumel is easily the best place I've dove so far. But I'm going to Raja Ampat on the Velocean (my first liveaboard) this January), so this could change.