I agree with all the posts (not a big adMob fan though, just google them and see problems people have had with them.), but also want to recommend picking up more than one. Not only does it ensure a good fill rate, you can basically split test them and see who has the best ad optimization for whatever demographic you're shooting for. Adiquity, TapIt, Chikita are cool. Stick with new ones, they're breathing more exciting life into the mobile ad space.