Google Adsense will always be on top of the Popular Adnetworks list. Because they provide very good CPM rates to publishers when compared to all other networks. May be some of the Adnetworks provide more CPM rates than Adsense, but they require High traffic, Top level domains & also they have bunch conditions to join the networks. This is the reason why google Adsense is on the top of the Ad-Networks list. Because google Adsense don’t look for any big traffic requirement. Even small publishers can join Adsense and they can earn good amount even with low traffic websites.
Some of the Most popular Ad networks offers very good eCPM rates for Publishers, But they require High traffic & some other conditions to join their networks. It is difficult for newbie website owners to join these Ad-networks because of low traffic.