I personally like and recommend Adsterra, PopCash, and PopAds.
With my experience - PropellerAds doesn't have a great CPM rate but they're decent to use, however Ad-Maven are a bunch of scamming thieving Jews as they still owe me $105.20 and won't pay up, it's been 6 months and I have contacted them multiple times... They seem all happy to help, but once I mention that I haven't been paid they go all quiet. I advise against using Ad-Maven.
I've tried a bunch but I have a combo of Adsterra, PopCash, and PopAds on my sites and I'm happy with those and highly recommend them. PopAds to a lesser extent because some of their pop ups contain viruses. It's all dependant on how much you care about your users.
PopCash have a great CPM rate, but I seem to get less impressions with them. Only 150 compared to 980 on Adsterra. Not sure if that's because there's less ads available. So that's something to take into consideration when choosing your pop ups.
But, these suggestions just make you aware of them, you should really try all and choose what you feel is best for you. Except Ad-Maven, don't try Ad-Maven.