If we're talking online, WolframAlpha is amazing. Used to use it all the time. Now I use Emacs Calc, since it's built right into Emacs (a keybind away), and it's pretty nice and featureful.

I like Wolfram Alpha a lot but google's calculator is faster and does a very good job with units analysis.

I open up a Python shell window probably 20 times a day to do simple maths…

I recommend https://insect.sh (you can install it through NPM to use it locally). Does simple maths well, does functions, does units, does conversions... pretty cool shell calculator.

the spotlight search bar also does math