This one gets my vote by and far. Not a whole lot gets done without this being true. I guess that's why it's been named "fundamental" eh?
I agree that the fundamental theorem of calculus should be included. There is also a generalisation called Stoke's Theorem which, while perhaps a little advanced to be called "fundamental", actually incorporates several concepts in physics and mathematics - including the fundamental theorem (and Green's theorem, and the divergence theorem), is used to derive equations such as the wave equation and the diffusion equation, and is of central importance in differential geometry, general relativity, and many other fields.