For shallow dive with lots of marine life and coral, definitely Hol Chan and Shark Ray Alley. One place you can dive and snorkel, and the other you can only snorkel. I can't remember now which one...
(Some may say they are crowded, but that is a relative term. There is a reason they are "crowded," and it is because they are worth it! I have never found them to be too crowded, and have been to both several times.)