Ironically builtin smartphone calculators are really bad, and one of the best ones you can download might be Graph 89 (a TI-89 emulator).

Rant/Aside: Smartphones (or at least Android) are just generally really bad at being... smart, especially out of the box. No dictionary? No thesaurus? To say nothing of built-in encyclopedia (e.g. Wikipedia). Calculator worse than the $1 scientific ones? It's astounding how obvious it is that they're meant to dumb people down and just sell you crap when you look at the complete absence of basic functionality anyone from 50+ years ago might expect them to have.