Most used
Most versatile
Classic go-to basics
Most powerful
Personal favorites
Biggest time savers
Etc...

Doesn't strictly have to be formulae. Might be conditional formatting, or whatever.

I need something to do at work that looks work-like, and brushing up on excel skills seems like a valuable thing to while away a few hours with.

This kind of thing is, of course, a double edged sword. I saved a chunk of time earlier by using the command prompt to create a .txt list of filenames and import them rather than typing them in by hand. Good practice. But stuff like that means you can end up getting jobs done a little too quickly. Anyway, let's get this thing underway...