During my bachelors, I was using a Casio FX-991ES+. It was a beautiful calculator compared to the hand-me-down Fx-82 VPAM. And pretty popular with the folks of my age that time.

During and after grad school, I was using Mathematica or NumPy/Sympy quite a bit. But it felt like using an overpowered system to do basic assignments. Think of taking a Bugatti Veyron for grocery shopping. I indulged myself with another physical calculator - this time a Casio FX-CG50 with a color display and python support. I use it whenever I do self-paced courses or reading the occasional stats/ML paper.