I've just returned from Cancun, Mexico.
If anyone's planning to go down there in the next week or so, you're in for a good time. The place was crowded with college aged kids, and almost all the females seemed to be wearing tight tops, making it very easy to pick out the more well-endowed ones.
Here are the nightspots I checked out:
Senor Frog's - This is a big place, apparently very famous, with a large dance floor and a stage, where the DJ encourages women to get up (and a large security guard prevents men from joining them) and dance. Some showing of thongs (during the "Thong Song") and some flashing of breasts as well.
Bar Del Mar - Located right across the street from Senor Frog's, this is a more laid-back place, with two open-air levels, suitable for socializing rather than gawking.
Slices - The smallest of the bars we visited, but one of the best. Here girls can get free shots for climbing up on the bar and dancing. It was here that I saw some of the bustiest girls (all in tight tops, of course).
CocoBongo - Another famous place, (see their website: http://www.cocobongo.com.mx ). This was the largest place, with a main dance area that probably held almost a thousand people. A very festive place, though the festivities are very orchestrated, with performers dressed as Lenny Kravitz, Vanilla Ice, etc, onstage lip-syncing to songs. We were initially attracted to the US$25 all-you-can-drink special. However, the place was so crowded, that it proved impossible to move. Getting to the bar took up to 15 minutes, and although there were many attractive women there, it was so crowded that it was nearly impossible to enjoy them.
To sum up: If you're wanting to see the largest number of women in the best environment, I recommend Senor Frog's. For a slightly more intimate atmosphere, Slices is a close second.
Hopefully this information will be of use to somebody. Hasta luego!
(Note to Spanish speakers: Yes, I know the tilde over the n in "Senor" and the inverted exclamation point before "Hasta" are missing - It's only becuase I didn't have time to figure out how to type them on my Linux box...)