I use BVCalc on iPhone/iPad/macOS since it has the best RPN mode of any calculator I have tried. It's the only calculator that shows you the algebraic expressio

I use BVCalc on iPhone/iPad/macOS since it has the best RPN mode of any calculator I have tried. It's the only calculator that shows you the algebraic expression (i.e., infix mode) next to the corresponding result value for each stack item, and it does this recursively as you perform operations on the stack! This helps you check for entry mistakes and makes it easier keep track of what each stack item represents. I still use an old HP48GX occasionally as well as well.