Don't know the best, but the biggest is Google. And, for me, I would need a good reason not to use Google. As for your results, do you have any statistics to share. I may be able to help you, but I need lots of information.
There is multiple questions to answer:
Are the right ads being displayed, are they being displayed in the right place/time?
Are they even being displayed?
Is the software code installed correctly?
How good is your understanding of your clients - what this means is, your app description attracting the right sort of ads?
If they don't like these ads, what ads will they click on.
And, getting back to your original question - does your ad supplier have the right ads?
Hope this helps. If you have more questions feel free to contact.