Which clocks are considered the most accurate?

Atomic clocks are definitely the most accurate timekeepers in the world. It is a device that uses an electronic transition frequency in the microwave, optical, or ultraviolet region of the electromagnetic spectrum of atoms as a frequency standard for its timekeeping element. Atomic clocks are considerably more accurate than quartz or any other clocks as they can be accurate to within a few seconds over thousands of years.

The first accurate version of the atomic clock was designed and built by Louis Essen in 1955 at the National Physical Laboratory in the UK. Other versions appeared earlier, but they weren't that accurate and were created to show the concept. Nowadays atomic clocks are widely used in different spheres, such as TV broadcasting and GPS navigation.