I am using and loving Solve by Pomegranate Apps. Multiple workspaces and advanced functions available a swipe away. It doesn't seem to be available anymore, and it looked like the company was hitting hard times when they dumped things like Trump quotes and other shit apps into the store. So, apprehensive to recommend because of weirdness but best calculator ever.
Edit: down arrows barely meant anything on Reddit and they mean literally nothing here. Fuck you and your annoying orange. Peace out, hugs and kisses.
My favorite was MathAlly. I still have it through some built-in android backwards compatibility emulator, but once it goes, it goes. They haven't been on the app store for years.