Hard light is the rarest gun in the game then thunderlord comes in a close 2nd
The only reason I use Hard Light is because it's actually rare. Not only is it rare statistically, I personally have never seen it in game other than the one I have.
I got hard light fully upgraded its horrible stat wise, but it one of the few guns for some reason looks actual next gen, I'm not sure why but the graphics on that one gun are better then the rest, its also one of the most fun guns to use I find.