The following chart shows the number of times various of my rules have been hit by comment/trackback spam on my Movable Type blog installation. I use MT-Blacklist to block such stuff, one of the pluses of which is that you can check how many times (and when) every anti-spam rule you have has been triggered:
Interestingly, poker is way out in front, and the first three items have more hits than the next twenty combined. Particularly intriguing, at least to me, was that the out of 4,500 items in my MT-Blacklist block list, only 800 of the links were ever hit again after being added. Junk URLs apparently mutate faster than common bacteria.