I too would recommend SexKey. My ratios were 1/15 the first half of September. I also do real well with
Free Net Pass , although my ratios are not as good. I was 1/63 last half of August, and 1/73 the first 2 weeks in September.
I have never done as well on any of the others, so I do a lot better selling the free passes then the paid, but it depends on your site, if you have a kickass tour, unique content, you might want to consider
Cyber Age Platinum Program.