It's going to depend on what offer you're running, and the countries you're getting traffic from. Some apps pay more than games, and vice versa. The countries you have traffic from also plays a huge role. As a general rule if I were looking for a network, I'd find out the offer you want to run and then use a site like Offervault to see which networks have it. They'll list the payout there as well so you can get an idea as to who pays the most. Once you have a couple of networks lined up then research them, or reach out to one of the AM's and talk to them to get a feel for who you're most comfortable with. Keep in mind that most CPA's have PPI offers as well, so you're not limited to PPI networks exclusively.