The format is quite new, so I think it's better to try different networks, to check which one is better.
And don't look at CPM but at revenue, because every network counts impressions differently.
For the start, you can find the biggest sites on your niche and geo on Similarweb and check what do they use. It's not a guarantee that it is the best choice for you, but at least gives some info.
You can check out EZMOB Mate! we have POP,Push, Banner, Native, floating Push, Java script, In-banner for adult and mainstream both.
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In all networks, you need good traffic to reach the maximum CPM! Currently, I run a Push notification with Exoclick, for the moment I earn 30% less which it was a month ago, my manager said I have to send from a better source to reach my maximum profit. But I'm invested more this month to get the best.. they really don't know the business!