PhysicsWorld has the list of the 20 Greatest Equation, listed in order of the number of people who proposed them.
The winner? Maxwell's equations:
... most votes were given to Euler's equation and to Maxwell's equations, which describe how an electromagnetic field varies in space and time. Although Maxwell's equations are relatively simple, they daringly reorganize our perception of nature, unifying electricity and magnetism and linking geometry, topology and physics. They are essential to understanding the surrounding world. And as the first field equations, they not only showed scientists a new way of approaching physics but also took them on the first step towards a unification of the fundamental forces of nature. A firm called Ocean Optics in Florida even sells T-shirts with Maxwell's equations on.
Link - via Microsiervos