Calculators are like ice cream flavors. We each like our favorite for our own reasons. The iTunes App Store write-up on each calculator app gives an accurate representation of the app. As a personal example, I am an HP RPN calculator man. The iPhone HP48 GX calculator app is a "perfect" emulation of the venerable HP graphing calculator. You may prefer a TI emulator or an original app that does things that only an iPad can do. Nobody knows but you. Search the iTunes App Store for "calculator" and select the one(s) that you prefer.