Fabrication of unique tapper ended GaN-Nanotowers structure based highly efficient ultraviolet photodetector is demonstrated. Hexagonal stacked GaN nanocolumnar structure (nanotowers) grown on AlN buffer layer exhibits higher photocurrent generation which significantly enhances its responsiveness towards ultraviolet illumination and leads to outstanding performance of the device. The fabricated detector display low dark current (~12nA), high ILight / IDark ratio (>104), fast time-correlated transient response (~433µs) upon UV (325nm) illumination. A very high photo responsivity of 2.47 A/W in self-powered (zero applied bias) mode of operation is reported. While in photoconductive mode, the responsivity is observed to be 35.4 A/W @ -3V alongwith very high external quantum efficiency (~104 %), lower noise equivalent power (~10-13 WHz-1/2) and excellent UV-Vis selectivity. Nanotower structure with lower strain and dislocations as well as reduced trap states cumulatively contributed to augmented performance from the device. The utilization of these GaN-Nanotower structures can potentially be useful towards the fabrication of energy-efficient ultraviolet photodetector.