It depends on your niche.
Because if you define "best" CPA based solely on how much they pay out, you're asking the wrong question.
Sure they may have a program that pays really high, but if your traffic isn't going to convert or if you get challenged for fraudulent email entries, then it would be worthless to you.
Start with your niche and look for CPA networks that pay. Test them out.
Even low-paying CPA networks that are tightly within your niche can still be good choices.
Why?
If they converted at a high rate, even if they're giving you a buck per signup, you may still make more money with them than with somebody else who may offer a lot more money. Still, the conversion rate is simply not there.
Also, the niche targeting may not be there as well.
Keep in mind the value that the ad brings to the lives of your visitors.
Now, this assumes you have a website.
But even if you're just promoting CPA pages using free traffic methods, you should still consider the factors I raised above because, at the end of the day, an ad is a form of content.
It has to deliver value.
Otherwise, people will not trust it, and you probably already know that if you don't trust a seller, you probably won't buy from that person.
It's that simple. I've yet to meet somebody who blindly buys stuff without first trusting whoever it is they're buying from.