What is your favorite isomorphism?

Question

By "isomorphism" I mean any structure-preserving map with a structure-preserving inverse.

(Please accept my advance apology if this question is out of bounds. I sense that it's borderline, but I'm hoping it'll be considered in better taste than the typical "What is your favorite X?" question. I think a collection of great isomorphisms would be interesting because of what isomorphisms uniquely have the power to do: reveal deep and astonishing connections between seemingly unrelated fields of study; open a channel through which techniques and ideas can pass between disciplines; collapse two or many problems into one.)

Answer 1

I like the idea that an $n$ dimensional vector space over the field $\mathbb{C}$ is isomorphic to $\mathbb{C}^n$

Answer 2

The natural isomorphism between a finite-dimensional space and its double-dual.

Answer 3

Isomorphism between the infinite spiral given by Riemann surface of $\log(z)$ and the punctured sphere.

Answer 4

To follow previous answers, I would say the isomorphism between a finite-dimensional vector space and its dual, even though it isn't natural (MacLane & Birkhoff 1999, §VI.4, from Wikipedia).