It depends on what you want.
If you want the one that converts, pick the one with high gravity and without any opt-ins on sales page.
If you want to hijack one's domain, pick one with low gravity and low competition on google.
I would recommend you to try it with the products with high gravity, because these are usually the best there. But you will have to drive traffic to your page by yourself because you won't be able to rank on google unless you are a big SEO expert.