Google AdMob, AppLovin, and Unity Ads are three of the most widely used mobile ad networks, and each has a slightly different center of gravity in terms of app category and format strength.
Where each network tends to be strongest
AdMob has the broadest reach across app categories and integrates tightly with Google's wider ad ecosystem. AppLovin has built a strong reputation specifically in mobile gaming, with mediation and user-acquisition tools built around that vertical. Unity Ads, unsurprisingly, has deep roots in games built on the Unity engine and strong rewarded-video performance in that context.
Picking one isn't usually the right question
In practice, most publishers run two or more of these networks simultaneously through a mediation layer rather than picking a single winner — letting the auction decide which network pays best for a given impression, rather than betting the whole waterfall on one network's rates.