Android - Ad agency that pay for impression [closed]

I was wondering what is the best Ad Agency that pays for every 1000 impressions.

I have recently released my new app (Tic Tac Toe Go) and have been using AdMob, however I recently found out that they do not pay for impressions and clicks only.

I would like to change this and was wondering what is the best ad agency to go for, ideally one which has a good fill rate and pays out on impressions.

Thanks in advance, Raj