WishList Member hands down.
I've tested nearly every membership application out there and Wishlist in conjunction with Wordpress is the best option.
There are other solutions of course that I have not tried, or that handle integration with other platforms such as Joomla, Drupal, etc.
But when it comes to converting Wordpress into a membership site Wishlist is the best option in my opinion.
It is of course a paid solution and in this case you absolutely get what you pay for.