Since college days I’ve always been an RPN (Reverse Polish Notation) fan, used HP calculators, and use an HP-like RPN calculator app on my iPhone. I like the stack, and the way you can see all the numbers you enter before adding them all up (or whatever) as a double-check before performing the operations.
Left-to-right entry always seemed to be missing info for double checking.
But the current version of the standard calculator on the iPhone is quite nice. You can enter long numbers and operands, in the middle of entering you can check something else (e.g. exchange rate) and return to the entry. The numbers shrink to fit. And what’s really cool is that you have a history of operations! You can look back over history of the the calculations you’ve entered and their results.
In older versions you needed to run it in landscape mode to access the scientific calculator, but now you can set that as your entry in portrait mode too, which is more convenient.
There’s also a built-in currency converter.
And there’s the “math notes” feature, which I still haven’t figured out. I probably won’t use that much, but I understand it can solve equations.