There's no universally "best" ad network — the right choice depends on an app's category, audience geography, and which ad formats it plans to run, and most publishers end up running several networks together rather than just one.
Match the network to your app category
Some networks have stronger advertiser demand in gaming, others in utility or lifestyle apps; a network's average reported eCPM is only meaningful if it's benchmarked against apps similar to yours, not the platform's blended average across every category it serves.
Start with mediation instead of a single network
Rather than picking one network up front, most publishers get better and more stable results integrating a mediation platform that can route requests to several networks and let real-time bidding decide which one pays best for each impression. That reduces the cost of guessing wrong on a single-network bet.