I'm an iOS dev.
This depends on your app.
For most situations with banner adverts I would go with Google Admob. Admob includes the ability to display iAd advertisements as well as many other companies. Look up Admob mediation in a search engine.
I had Admob Mediation displaying iAd to the UK and USA, and Admob banners everywhere else in he world. However iAd has very poor returns for me, I've gone fully Admob and have no regrets.
If your app is a game, you have further options. You could choose a company like Applovin which does banners suited for game apps. They have good eCPM, but for me low RPM (the fill rate is low).
You could think laterally - display video advertisements between the game over screen and the title screen, or perhaps interstitials. (Vungle, AdColony, Applovin, Chartboost).
Me - I have a series of non game apps. I go with Google Admob exclusively for banners - the return is good. I also have interstitials in there from Applovin. I also have an IAP to allow the removal of adverts for those who don't like the adverts. I also link to my other apps from within my app and have iTunes affiliate links. There are many many income streams within an app.
I have game apps also. These have not been as successful, there is much competition. I use Applovin, Chartboost and AdColony within these to produce good revenue.
I hope this helps.