I'd have to disagree with Wiilldd1...I don't think Cozumel is the best location for a beginner at all. Don't get me wrong, I love Coz, it's amazing; I've been there three times, and would love to go back. I just don't feel the deep walls and drift diving are necessarily right for someone who is starting out. Drift diving does NOT make it easier if you aren't used to diving in strong current...and the current can rip you along pretty good in Coz...someone who is not a strong swimmer could easily get separated from their group if they aren't able to control where the current takes them, or how quickly. (Yes, I've seen it happen.)
To be sure, there ARE some shallower reefs (shallower in the 40-60foot depth ranges) but many are closer to 90+, and the walls can easily drop deeper than 100 feet. (They actually plunge several hundred feet straight down, but the dive profiles tend to keep the maximum depth to 100-120 feet, depending on the outfit you use and the people you are diving with.) That may not seem like a big deal to those of us who dive that depth regularly (and those of you out there who routinely go much deeper) but for a beginner it can be very intimidating, to say nothing of the fact that their bottom time will likely be extremely limited, and as new divers they are likely wanting to spend as much time as possible under water.
Now, I should admit that I haven't actually spent any time diving in other Mexican locations, so I don't really have a first-hand reference for comparison. If you were just to ask me what the diving is like in Cozumel, I'd say "phenomenal, outstanding, tremendous...and Cozumel itself is absolutely wonderful." But the specific question was about a good site for BEGINNERS, and I honestly think Coz is a better destination for the diver with at least moderate experience, especially those with some previous experience diving in strong current and at depth, for it to be enjoyed fully.
I'll be in Cancun during the week of March 14-21, and while I have not dove there before, reports seem to suggest it would be a better first-time trip for the new diver. Shallower reefs, less current, with plenty still to see. Again, that is based on what others have said, NOT first hand experience talking, but I will be sure to give a trip report and post some of the video I get while I'm there.
So, by all means head to Coz if you like, I can promise you won't be disappointed, but be aware of what you're getting into, and whether it truly suits your experience and comfort level. Just my two cents.