I actually made my first conversion with PropellerAds. And after testing few other push networks I am still here to stay because of their traffic quality and best customer service (when you open the chat window you never wait longer than 2 minutes). You get plenty of options to finetune your targeting most important in push in my opinion choosing (User Activity) and Premium traffic. In addition they always have very good webinars to get you going and a seperate "feature suggestion" section.