For me, clickbank is the best. They have a lot of products in a wide variety of niches. However, their affiliate links are rather clumsy and so I strongly advise you to use url shorteners.
I use Wealthy Affiliate platform for tools, community and quality training. They have literally everything you need to create your own brand from the ground to the top lists. It is a platform for affiliate marketers of all levels and I feel blessed I found it. I definitely recommend it to everyone that wants to learn, put some effort and become financial independent. It costs $47 per month, which is very little if it is to compare it with other platforms that don't even have such a great and new technology. They are the best in affiliate marketing from 2005 and they're still rolling and change people lives.
I forgot to mention that they offer you 20 free lessons that will help you understand the whole process. You will not need a credit card when you sign up. There are also people who make money with the free membership, so you don't have to upgrade if you don't want to, no one will force you.
I recommend it with all my heart! Also, if you want to know more, I wrote a review and you can read it here: Best Way To Learn Affiliate Marketing - Wealthy Affiliate University. It's not that kind of review that presents only the good parts. I offered all the info I could, based on my personal experience with Wealthy Affiliate. You will find both positive and negative aspects.
I wish you all success!