I can't really pick between the equation giving the Hamiltonian of a system, or the Euler-Lagrange equation. I'll go with the latter, because I learned it first

I can't really pick between the equation giving the Hamiltonian of a system, or the Euler-Lagrange equation. I'll go with the latter, because I learned it first. Learning about these in my classical mechanics course suddenly made an entire class of problems which was previously incredibly tedious to solve fall away into trivially easy forms. I got a similar feeling to when I learned about using Laplace transforms in my ordinary differential equations class.