Depends what age/year/level?
And if you're looking for something that teaches, or just practice.
Maths Seeds (by ReadingEggs) is good up to about Y3 level. Every session has a fun little lesson explaining new concepts, then some practice (in the form of various little games), then some practice of previous topics. Good if you're looking for 'teaching' but not quite enough practice included (for most children).
It's by subscription. App only for iPads, not Android, but can be used web-based on Android tablets.
Squeebles apps (there are several) are good for practice of the four main operations, and for memorising times tables. Doing enough practice unlocks elements for the games that form the incentive. One-off payment.
Don't know if iXL can be used on a tablet, and haven't used it myself, but from what I hear / read it provides lots of practice, and follows the NC precisely. Subscription, with some free questions/day.
Dragonbox apps are IMO fantastic but each one only for one topic. We have used Numbers (great for number sense), Big Numbers (column addition/subtraction), Algebra 5+ (intro to algebra), Algebra 12+ (bit more advanced), and Elements (geometry). I'd recommend each one of them.
DS also enjoyed 'Slice Fractions' an app/game all about, well, fractions.