I think anywhere but Cabo is a good choice, with the exception of Cabo Pulmo and especially the Gordo Banks.

I haven't personally been to Cancun, so hopefully others will chime in, but from the photos in the below links, it smokes Cabo.

Here are some Cancun scuba links:

http://www.scubacancun.com.mx/

http://www.cancundiving.com/

http://www.cancun-aquasports.com/

http://www.cancundivingadventures.com/pictures.html