Preferably atomic clock since it works on decay of atoms. Decaying of atoms is not affected by any property known in science. Temperature, gravity, etc etc any anything else does not affect the rate of decay of atoms. But if the clock starts moving at a certain velocity, then this value changes. Go near the speed of light and the rate of decay of this material compared to rate of decay of the same material at idle or lower velocity would be different. This property is defined as time dilation.