Calz

$4.99 FROM WapleStuff, waplestuff.com NEEDS iOS 10 and watchOS 4 or later

Calzy is the iOS calculator taken up a notch. You can bookmark calculations for future reference, drag and drop values, and browse your calculation history. You can even rearrange its layout.

Its Watch version is well designed for the small screen, and the app looks great no matter your device.