hbar prolly I guess.
It's kinda funny though cause in particle physics you use hbar, c, and 2*pi a lot. Keeping track of them all is kind of lame, so my prof started a lecture by writing the following up on the board:
Let hbar = c = 2*pi = 1
Actually though, you CAN make the change to hbar = c = 1. As a consequence, you measure time in centimeters, mass and energy in inverse centimeters, the unit of time is the time it takes light to travel 1 cm and the unit of energy is the energy of a photon with wavelength 2*pi cm.