I thought Uxmal (sounds something like Ooosh-mal) was by far the best in terms of both the size and quality of the ruins, and the lack of other people there. In fairness, we were there relatively early in the morning.
http://en.wikipedia.org/wiki/Uxmal
Chitzen Itza, at least IMO, was a bit Disney World - so packed with tourists and people trying to sell crap that it almost ruined the experience.
edit: I'm not sure how far Uxmal is from Cancun. Maybe too far.