As far as I am aware, frequency is the most accurate physical property we can currently measure. This has led us to very high standards of experimental verification of e.g QED predictions and the CMB spectrum.
EDIT: as is pointed out below, the second is a defined figure and time should not really be listed here. Please take measuring frequency as a distinct matter.
Out of pure curiosity, I am wondering what the next physical property, without involving energy or time, is it that we can measure most accurately?