The question is wrongly posed. Waveguides can be and are used for any wavelength from ~50 cm ( ~600) MHz upwards to mm-waves and submillimeter waves; the advantage is a very low loss.
Other guide lines like coaxial and stripline are also used but generally have a larger loss.
There are also "overmode" waveguides with a cross-section measured in tens of wavelengths. Such waveguides are called beam-guides and can be seen as feeders of large satellite-communication antennas (chimney-pipes). The reason is even lower loss, very important for transmitting large power or for very low-noise receivers.
The X-band is particularly used for radars, terrestrial as well as airborne. Waveguides offer a low loss, so the microwave power generated in radar transmitter is not reduced on its way to antenna; also weak returns are not attenuated on their way to the receiver.