What is the best concept in CS/Math you've ever learnt and why?

So many to choose from! I'll stick to some small practical examples.

If you've got a queue of any form, you can throw Little's Law at it.

"the long-term average number L of customers in a stationary system is equal to the long-term average effective arrival rate λ multiplied by the average time W that a customer spends in the system. Expressed algebraically the law is L = λW"

[1] https://en.wikipedia.org/wiki/Little%27s_law

I frequently bring up the Secretary problem when talking to people about dating and job hunting:

(1) estimate the size of your candidate pool as N, (2) reject sqrt(N) candidates, (3) choose the first candidate that is better than all others.

[2] https://en.wikipedia.org/wiki/Secretary_problem

How to estimate things. That is the best thing in Math I have learnt. It is known as Street Fighting math or Fermi estimation and many other names. One book that is close to my heart is:

Street Fighting Mathematics by Sanjoy Mahajan.