THE function. What the 1 is for all fields the e-function is for all functions. Euler was a true genius. And way underrated because half of his definitions even don't carry his name.

https://en.wikipedia.org/wiki/Karl_Weierstrass was the first to find a function provable mightier than it.

99.9 % of people (the rest isn't significant) don't need anything mightier than it though. :)