"Okay. I get it now."
For me, Operation Math is an app that I can count on again and again to help my students master those difficult basic skills that we all need to know in order to easily calculate more advanced problems (think order of operations and solving equations).
Students are able to pick what operation(s) they are going to focus on (I usually tell my students what to do, since sometimes they pick something they are already pretty advanced at).
This app has students sign in as a special agent, then solve math problems by typing in the answers on a wristwatch. As students advance through the app, they unlock other 'missions' (more math problems). But even my pickiest (is that a word???) students will play this app without complaining!
It does cost $2.99 at the app store, but I think it is absolutely worth it! (I think you can also download a free trial version)
So, what's your favorite app?