Google recently changed their T&C so that indie app stores can't use Admob anymore.
InMobi is the best alternative to admob for mobile display advertising . The reach is truly global alternatively you can use millennial media
InMobi is good, but we had 2x more volumes with MisterBell on a particular launch last year.
From my point of view you should usually stay away from comments that just state "the best is xxx". there dis no best, it all depends on your niche/target, user flow/experience and desired formats. There are a ton of ad networks, not to mention ad mediation and RTB to choose from.