I think every pragmatic programmer or aspiring code guru needs a core programming challenge that they return to whenever they want to try something new, like signature tune a guitarist will play on every new guitar to see how it fits their style.
My favourite pattern is The Mandelbrot Set because it’s a nice way to check the main features of any language : looping, branching, and creating complex structures, as well as adding a graphical level to start looking at the surrounding libraries. It’s also a neat optimisation problem, and each language I’ve used lends itself to slightly different optimisations.
So what’s your workbench? Do you build a unit-testing framework? Or a shopping cart app? Or do you turn every language into a LOLCode parser?