Hey results, wanted to chime in on this one. Our site will help you a little with finding the most popular (thanks Dom). Paradise Poker has been getting some good reviews lately.
If you are trying to find some information on the most popular casino sites on the web, you may want to take a look at http://www.alexa.com. They will give you traffic rankings for various sites, which would be an indicator of who is most popular. Or at least who is a better marketer.