You'd probably earn the most with affiliate programs of casino's, but I'm sure there are a lot of good non-gambling related affiliate programs too.
I can't seem to find a good list which summarizes all the affiliate programs by earnings/percentage, but here's a plain list of most services that offer affiliate earnings:
https://en.bitcoin.it/wiki/Bitcoin_Affiliate_ProgramsI don't know how up-to-date that list is though.
It depent on the traffic - im looking for good services i can used for mainstream visitors.
Also good partnerprogramms and ref-options.