We got a bunch of jokers in the comments. Technically correct is the best kind of correct.

The constants which get the most use are Euler's constant e and pi. If you don't know about Euler's identity, it's e^i*pi = -1. It's kind of a meme at this point just how MINDBLOWING Euler's identity is, but it is just one output of Euler's formula. That formula gives a nice way to describe any point on the unit circle in the complex plane. So beside e's many straightforward uses, it is a bridge between things that are not obviously connected.

Lots of problems have to do with circles or can be translated into a problem about circles. So pi crops up a lot as an algebraic necessity of discussing circular things, but I'd say e is more actively used as part of the process of getting to answers. Aliens may use a different circle constant but everyone knows about e. I think the best Voyager-like message to prove basic species intelligence is probably the binary representation of e.