AdSense
Google AdSense is the best ad network. If you can get approved by Google then you won't find any ad network better than this. It's the biggest advertising company out there. The good thing is that they do not monetize adult content. Something they don't want to monetize then you might not get approved. Their CPMs are very competitive as always.
Ezoic
They are great but they do have a minimum traffic requirements. If you're new, chances are that you won't get approved. So most small businesses or small publishers with some heavy traffic tend to go with this company. The CPM is slightly higher than the average and by slightly I mean considerably higher than the average. The types of ads they have is display ads, native ads, in line ads, pop-unders, and they are considered to be the future of ad industry.
ExoClick
The only reason I put exoclick on this list is because of their platform. I think their platform is simply amazing they have every single type of ad. You could adjust the settings to make it Google friendly. What's bad about them is most of their ads are adult. Most people don't want to advertise that type of ads in order to keep their website as professional as possible.
Exoclick has stuff like banners, pop-unders, instant message, interstitial, mobile ads, vast (before videos ad), native ads, exit widgets and push notifications. Tthe thing I didn't like about exoclick was their CPM. There CPM is low infact not only is it low but the ads are also mostly adult. They have quick approvals, you could get in and get started with exoclick right away.
Adsterra
You can make money off the affiliate program by them. They are legit, there is nothing wrong about them and they approve even blogger websites. They do have adults ads but have normal ads as well so it's a really good ad network to get started. If you're just starting with ads and want to learn how to insert ads then adsterra is a great place to start with. It's easy to navigate and you could understand it pretty. There's no minimum traffic work requirement so you could start right away. There CPM is reasonable, not the best but it's reasonable and another great thing is they also have vast ads so you could put videos on your website and put ads before the video with adsterra just like exoclick and Google AdSense.
Propeller
Propeller ads serve you whether you have no traffic or whether you have a lot of traffic.They are willing to give you a chance even if you don't have a minimum traffic requirements. You could hit the CPM of 100 on their pop-unders. I'm not saying it's a lot but the CPM was 100 so that means every 0 visitors worth that CPM. And it's just an average anyways so propeller ads has multi tags in page, push notifications, on click, pop-unders, interstitial and direct link