ok...assuming you are not just baiting me here....(or taking the bait if you are)
Well, the Schrodinger is probably one of my favorites, which I mentioned earlier. Oddly enough, my research now centers around applying this equation to solve quantum chemistry problems. I didn't have a clue my research would be in quantum chemistry when I chose this equation as the key equation for the cake (I know...pinnacle of geekdom, like I said). It can be solved applying eigenvalues, which is pretty neat. Also, its application can answer questions about how electrons behave at the atomic level - which is pretty amazing to me.
I would say that perhaps my most favorite is e^(i*pi) = -1. It is pretty amazing. It contains so many basics of math - the natural logarithm, imaginary number, pi, negative number, and an identity. It is pretty much the bone-in filet of math identities IMO

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I am also a big fan of f(x) = lamda*x*(1-x), which is a simple description of chaos and bifurcation theory. I'm a big meteorology buff and Edward Lorenz did some awesome work with chaos theory development from weather studies (still the basis for why weather patterns can't be predicted outside 10 days or so). I kind of fell into chaos theory as a result.
Although it is often overused, e = mc^2 tells a pretty amazing story. I am a HUGE nuclear energy and nuclear history buff - and that equation gave the answer before the scientists new the right questions to ask.
I'll try to find a picture of the cake...I know we have it somewhere...