E=MC^2
KE=1/2MV^2
Pv=nRT
A=PiR^2 (But no, Pi R Round)
There are some cool astronomical ones involving gravity that I can't recall, also; G=m something. Newtonian stuff, very elegant. Also the aerodynamics equations for lift & drag are remarkably simple also IIRC. Thermodynamics has some great ones too.
The most incredible stuff I remember involves the application of natural and base 10 Logarithms, and how well Calculus can describe the physical world. I don't understand it, but it works.