I will go with pi. I have pi memorized to about 55 places, give or take a few. (I need to work on that, after seeing 'The Life of Pi'

)

But tau gets used in a lot of places I run into. For instance, the electronic reactance formulas: Xl = 2piFL and Xc = 1/2piFC, two of the most important equations in electronics outside of Ohm's law. (F is frequency in Hz, C is capacitance in Farads, L is inductance in Henrys, Xl is inductive reactance in Ohms, Xc is capacitive reactance in Ohms) Closely related is the formula for resonant frequency F = 1/2pfSQRT(LC) Tau, as it were, shows up in a lot of electric formula because it has something to do with the complete cycle of a sine wave, which traverses through 360 degrees, or 2pi radians. 2piF in electronics is often refered to as lower case Omega, and is given in radians/second.

Then in quantum physics, is Planck's constant, the basic size of a quantum of energy. It is constant usually given as h. But like Tau, there is a shorthand for a frequently used derived valur of Planck's constant that actually involves tau. The 'reduced Planck's constant' is given as h-bar (an H with a little line drawn across the ascender), and the formyla is h-bar = h/2pi or h/tau.