No dimensioned measurement can be made more accurately than its corresponding SI unit is known. Thus the measurements with the smallest uncertainty are those of frequencies as the second is the most precisely realised unit. The record holder changes but at the time of writing the most accurately measured physical quantity is the rubidium hyperfine frequency (known to 2.4 10^{-15 [1]}) This measurement represents 10⁴ improvement on earlier measurements. For many years the record was the measurement of the hydrogen maser frequency (known to 2x10^-12). Recently techniques of optical frequency measurement have improved and optical frequencies such as the hydrogen 1S- 2S frequency (known to 3.4 in 10^-13) are being measured with increasing precision.

There are other more accurate null measurements which can give information on the fundamental symmetries of physics. Examples of such measurements are the decay rate of the proton, or the mass of the photon, and comparisons of Josephson junction voltages. The mass of the photons is estimated to be < 2.1x10^-48 kg a much smaller mass than could be measured directly.

See Flowers, J. L. &amp; Petley, B. W. Progress in our knowledge of the fundamental constants of physics. Reports on Progress in Physics 64, 1191-1246 (2001).

1. Sortais, Y. et al. ⁸⁷Rb verses ¹³³Cs in cold atom fountains: a comparison. IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control 47, 1093-1097 (2000).