LaTeX
I know most of you use the rich text editor from overleaf (if you use LaTeX at all), but learning proper LaTeX coding and design will set you far beyond your pears when applying for jobs or in research. LaTeX makes it so easy to cite things making it practically a requirement in most academic settings.
Tikz
I use TikZ for drawing a lot of the simple diagrams in lectures and labs. It has a large learning curve but it’s so nice to have