There are literally hundreds of ad networks out there. I have a list myself of nearly 1000.
There is no “best” network. The ad network(s) you decide to use will depend in part on what your site is about and also what you want to do.
For instance, there are many networks catering to adult (meaning of a sexual nature) traffic. If you are selling bicycles, these would not be a best fit for you. But it might be if you sell adult toys. Other networks may specialize in different niche audiences. I know of one promoting itself as a christian network and if I recall, another catering to muslims.
Some networks have only certain kinds of ads, video for example. Some have a rather small network of sites. But that does not mean it's bad. You may want to push your ads on a certain site because that's where your audience is. In that case, whatever network the site uses, that's a good network for you.
There's of course the more general networks like Google Ads, Bing and Yahoo among others. They are the giants of the industry and you generally can't go wrong with them.
But no matter what networks you use, implementation is key. Simply giving them money to show your ads without a plan and understand how these networks work and how to advertise in an efficient way is just giving your money away. At the end of the day, you want some return on this investment.