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I don't know if it's the best but I use calculator pro. Not sure of the cost because I got it when it was free. BTW, there is an app discussion section here and you might get better feedback there.
 
I don't know if it's the best but I use calculator pro. Not sure of the cost because I got it when it was free. BTW, there is an app discussion section here and you might get better feedback there.

My bad, I was looking in the iPad section for an app forum, and I didn't see one.
 
Free42
Full emulation of the final (and best IMHO) HP RPN calculator (HP-42C).
Though it is not free on this platform...
 
If you're familiar with the HP 48, m48 is free and works very well. m48+ is pretty expensive but it emulates a bunch of calculators.
 
PCalc

I tried a few and settled on PCalc. Lots of nice options for viewing, and can go conventional or RPN.
 
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