Selecting an advertising network for your content website

If you have a large number of websites monetized by advertising, finding a suitable ad network is  important .
Online advertising can be classified into 2 major categories:
1. Pay per click (PPC) advertising
2. Display advertising

PPC advertising is vulnerable to click fraud and there are only 2 major companies who can dictate the price of clicks.  Some of the second tier PPC search engines like Exoclick verify your account only after you have generated 100 clicks. There is little transparency for the publishers and advertisers, so I have been focusing on Display advertising.

I have been experimenting with different display advertising programs / networks. Ideally the ad network should:
1.Count all impressions and offer detailed statistics.
2.Offer a good CPM  (for thousand impressions) rate for traffic worldwide. Currently, most networks offer a good CPM rate only for tier 1 traffic (USA , Canada). I am getting a CPM of just US $0.04 for UK traffic.
3.Pay through Paypal on time.
4.Offer Net15 days payment terms
5.Be free of spyware/ malware / redirects.

I am using about 10 networks at present and have not received payment from many of them since I have just started using them. Once you find a reliable ad network, it can be a good source of passive income.