I'm about to head to work, so I had to skim through these posts... with that I'd add my vote to the Key Largo line.
Key Largo is good for beginners for several reasons...
1. Shallow, hard bottom with little/ no current... The reef is in the 20-40+ foot depth range...
2. U.S. Soil... good should you need to use your insurance... I'd take my insurance in the U.S. over any rider/ policy in a different country. With that DAN Insurance is a good idea, no matter where you travel, but especially if you leave the country, IMO...
3. Native English speakers, remember you'll be learning from, and communicating with the instructor A LOT.
4. Ample Dive Shops (this is nice if you opt to buy your own gear)... I recall a recent thread of a diver looking for fins (maybe it was a mask) in Cozumel... There were a few recommendations for a place that might be able to help, but for such a dive destination I'm surprised there wasn't one 'go to place' to get what you need. Being a new diver you might find your mask doesn't fit well, or you want a shorter hose on your primary, or a different mouth piece. It'd be nice to have a week or two to get your set up dialed in for reasonable comfort before you head out to some far flung destination...
5. It's pretty cheap, fly to one of several S. Florida airports, rent a car, and in a few hours you're in Key Largo. Hotels are reasonable (~$100+ night), food can be reasonable (grocery shopping, SubWay, etc.), or expensive if you chose. Additional dives are pretty reasonable too, around $30-$40 per tank...
6. If you can fit it in, hit Blue Heron Bridge, a few hours drive north of key Largo... Ranked as one of the best dives in the country... I haven't made it there yet, but I'll be blowing bubbles under the bridge in a few weeks time

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With that, I'd recommend Rainbow Reef. It's the only dive shop I've used in K.L., but they've been great.