Search for "42s" in the iOS app store. It's an extremely good emulation of the classic Hewlett Packard 42s real-world calculator, now updated to 64 bit for iOS 11 compatibility. Note: like it's real-world counterpart, it is RPN (reverse Polish notation) entry.
For a more traditional algebraic entry calculator I like RCL-59, which is an emulation of the classic TI-59 scientific calculator.