system
A chat on subreddit /r/wallstreetbets
human
VLOOKUP, SUMIF (and the other operator based IF functions such as COUNTIF and SUMSIF), and IF are the three most important formulas you can learn to be better at using Excel. They elevate your game from a novice fumbling around with unwieldy spreadsheets to an actual user of Excel.

gpt
I think absolutel vs relative referencing is a game changer. Once you know how to do that it opens up so many possibilities.