Euler's formula. This one formula is the most practical formula I've ever encountered. Once you learn it, you will never stop using it. The Schrodinger equation

Euler's formula. This one formula is the most practical formula I've ever encountered. Once you learn it, you will never stop using it. The Schrodinger equation is a lot of fun, but you will likely stop using it if you stop doing quantum mechanics. Maxwell's equations are beautifully simple and tend to appear in many other contexts other than classical electrodynamics. For example, in the absence of charge, Maxwell's equations describe free waves -- which can appear in many contexts other than electromagnetic fields.