Stack Calc is the best RPN calculator. It is beautiful to look at, simple and solid to use, and adds a few "why didn't I think of that" features to the old RPN calculator.

OS X's calculator has an RPN mode, but for some reason iOS's calculator doesn't have an RPN mode. Stack Calc fills this void, and then some.

The stack is interactive. Tap an entry on the stack to move it to the bottom. You'll notice no X-Y buttons or Stack Roll buttons. Just tap the one you want to use.

You can tap and hold any entry on the stack to bring up more options, i.e. copy the value to clipboard or remove that item from the stack.