The right Clickbank product is the right product that fit with your strategy. If you want to run affiliate marketing strategy, than it does not make sense to purchase ppc course strategy. As you want to sell the product, you can choose product with the high gravity, because it will sell well. Good Luck
lots of categories have unlimited opportunity for making money. There is no one best category because like rudyarya said it all depends on your marketing strategy and how you will approach the niche. Some of the bigger money making categories are health and fitness, make money online, and forex. Pick a catergory that interests you and you will have a better chance of success with that.
Usually best categories are health&fitness. money making programs and security softwares! Also depends on you and your capabilities to promote this products because competition is pretty high!
Choose only the category you are confident of promoting. The category that you think you are an expert or can become an expert. Because people like to buy things from people who know about the subject matter. And most people today are smart enough to determine whether someone is an expert in a subject or not.