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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.
 
HP 12C

I've been using the hardware version for about 40 years. it's wonderful to have it on my iPad and iPhone
 
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