I tried several but the only one that's worth it is admob.
As backup (when admob is blocked (China))/cross promotion you can use Tappx. (ref link in my signature)
And every 2 or 3 months I have a payout there aswell which doesn't sound as much but more than enough to cover some licenses on a year.
most others are Pay Per Install only so you can show a lot of ads without earning anything.
some also have a extremely high payout thresholds and with high processing fees.