You need good vendor for marketplace. Check out the vendor and their products and read-out his website that the website is good for your product or not. Having good website of your vendor is also important.
Choosing the best affiliate marketing marketplace isn't important than:
1- Find your niche.
2- Choose your product.
3- Measure competition.
3- What is your traffic source.
4- Then Pick the chosen product to promote using Offervault or Odigger.
Some good affiliate marketing programs: Amazon Associates, eBay Partner Network, Walmart, Commission Junction, Impact, Awin, TradeTracker, TradeDoubler, LinkConnector, AffiliateFuture, ClickBank CleverBridge, Rakuten Marketing, Shareasale, Flexoffers, Jvzoo, Travelpayouts, and MarketHealth.
There are plenty of Affiliate Marketing Marketplaces, such as:
Share a Sale
I believe that Share A Sale is great for Influencers and people with a following, Clickbank is good for overall everyone when paired with good strategies.
Awin I heard well about but never really used and CJ has some of the best brands out there to promote, for I know it's if you also have a following.
Make sure you do your research and I also have a Free ClickBank Course, which you can check out below if you feel like it.
It's really powerful and delivers a great amount of valuable information that not everyone might know about.