Which is the most commonly used detector in High Performance Liquid Chromatography and why?

UV detectors , because most of chemical compounds especially which gets into phahrmaceutical industry have absorbance responce related to uv radiation with in the range of 190 nm to 370 nm 

UV-visible detctor is most commonly used dector with HPLC because most of the organic compounds contain chrompohores and double bond which show electronic transitions in the region of 190-380nm.

Photo Diode Array (PDA) is the preferable detector in HPLC. 

One can extract the wavelength they want and compare the spectral purity to identify the targeted components.

Ultraviolet detector, as it has wide range analytes suitable for UV detection typically contain unsaturated bonds, aromatic groups, or functional groups containing heteroatoms, which contain π* and σ* nonbonding orbitals into which electrons are promoted to absorb the incident energy. These nonbonding orbitals contain a wide distribution of vibrational and rotational energy levels that lead to a distribution of absorbance energies and therefore spectra with broad, rather than sharp features.

Analyte concentration can be determined from the Beer-Lambert law (equation 1): 

A = ε · c · l       [1]

where ε is the molar absorptivity coefficient (dm3 mol-1 cm-1), c is the concentration (mol dm-3), A is the absorbance, and l is the pathlength of the light through the flow cell (cm). If the molar absorptivity coefficient for the analyte is not known, then standard solutions of known concentration can be used to determine it or to calibrate the instrument (concentration versus absorbance) response using a calibration curve or response factor.

mostly used uv vis detector

Photo Diode Array (PDA)

UV

Flourecence

uv-visible detector because of auxochrome or chromophore present in molecule 

UV detector because it is easy to install and operate and it can detect any molecule with a chromophore: any aromartic group has a large absorbance and it is well detected, a group containing conjugated double bonds ore amide bonds are suitable.  

UV Detector because of broad range

DAD Detector, wide range of organic molecules and compounds contain chromophores which respond to uv light.

DAD detector is also cheap and easy to maintain