Expert opinions? Which strain of HPV increases the risk of anal/rectal cancer?

Most risk with 16/18. Squamous carcinoma of the anus is most often mediated / caused by infection with high risk hpv (in both men and women). There more than 10 types of hpv that are 'high risk'. Of those, types 16 and 18 are most strongly associated with progression to invasive cancer. In general, rectal cancers are not caused by hpv infections.