The best are usually the health and fitness niche. Look for products that people really 'need' a solution. Eg, 'weight loss', 'acne', 'get ex back' etc. Also ideally products with gravity around '100'. shows that it is in demand and also the competition is not too much that you can't sell.
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highest gravity can't mean the best selling products. you should analyze and promote the best ROI products.
I find it hard to sell Clickbank products generally nowadays. Those with high gravity usually also have very stiff competition for the keywords. Anyway, I try to target those with gravity about 80 to 100. At least, for those products, you will have less competition. And you won't find every other website in the niche promoting that same few ebooks too!
Anything that sells "hope" or fills a void. Since we're coming into the new year, definitely look into the weight loss niche. Loads of people make the new years resolution to loose weight. Gym memberships ALWAYS jump in Jan and Feb.
people are making money with all sorts of product, but as someone else said, there is no best, you just pick one, do some good keyword research, and build a review site around it, pre sell people and send them to the sale page, and keep your finger crossed that they buy from your link.
It's all about the kind of traffic your website receives. You need to find a niche that you are interested in, build a site around it, and market the corresponding products! Many niches and sub-niches are profitable.