I think there is no good answer for your question. What works for one might not work well for the other. It depends on your copywriting skills too, and the quality of the traffic you drive to the product's website. With super-targeted adwords traffic you can reach 1:10 conversion rate (well it's not easy).
You probably will not find the best "converting" clickbank product. However, you can browse through the marketplace and look at the gravity index to find the most "popular" product being pushed by affiliates.
It depends. Browse through the clickbank marketplace to see gravity and popularity of a particular product. It mainly depends on the personal interest. Go for a niche product, promote well and just wait for results.
High gravity sites are probably not the best choice to promote. If alot of people are already promoting the program, you can bet the PPC price for matching keywords is sky high, the search engines are already crammed with pages of other affiliates trying to sell the same thing, and competition will be stiff. Instead, try finding brand new programs with no ratings. These haven't been exploited yet, and if you can find one that will convert, you are at the "ground floor". Problem is, if it has no ratings, you can't be sure it is going to convert. So read it thoroughly, and decide if you can make it work, of if makes you want to buy.
I don't think anyone is going to answer honestly to that question. It would be like saying "Sure, I will give you some of my money!"
In my case I have been promoting the main page of our clickbank mall and visitors search or browse through it and purchase whatever they want. And, I get sales from many of the products in clickbank. So, what I understand is most of the products have good income potential. Choose the products according to your interest and past experience etc.