I would say since you are looking for Impression Based - you are going to want go with the optimizers like CPXi, Marimedia, YBrant, etc. They all payout RevShare with an eCPM.
You could also look for a straight CPM network like PremiumCPM which doesn't and isn't considered an optimizer so its straight CPM even though you get paid an Average CPM (eCPM) because of different campaigns having different rates.
Also - Remember support. Most of the ones mentioned above, Google, Chitika, CPXi, Marimedia, YBrant, etc they are all limited support. Within business hours, usually email only. blah blah.
Support is extremely important. PremiumCPM provides 24/7 Support via live operator, email, skype, and phone. Which is something no one else in the industry is doing right now.
At the end of the day you have to balance Earnings Vs. Support. But as a wise man once said, good support leads to better earnings.