The Primality App has a clean and simple, yet powerful, function: Given any integer, the app determines either that the given integer is prime or else it provides all prime factors of that integer. Very clean, very simple, focussed.
The app operates in the range of 0 - 999,999,999,999.
A popular feature of the app is its ability to determine the prime ordinality of each prime found, such as observing that 97 if the 25th prime. (However, this particular feature is available only up to the 100,000th prime, which is 1,299,709.)
Requires iOS 13 or higher.