Favorite Physics Equation...

My fave has always been the calc for Kenetic Energy.

But I use Mannings equation a LOT....

When in second semester physics we went from maxwell's equations to the wave equation for light.... it was a near religious experience...

phenomenal.

-B 

Being more of a math type than a physics type I don't get to put much into actual practice and so "favorite" is sort of relative.

Now if you drop the "Physics" from the question, then I have a better answer:

Crytography is something that I work with quite often in my profession and aside from the obvious RSA Equation and Diffie-Hellman Key Exchange, my interest is really in Eliptic Curve Cryptography.

Prime Factorization methods also held my interest for quite a while and still do in terms of developing more efficient algorithms for performing Quadratic and Number Field Sieve Factorizations of very large numbers.

ETA: I do take an interest in Physics Equations if the derivation methods are unique or clever.