Red Death has been sold twice by Xûr (let's say once because few people could afford a Primary the first week), so I doubt it qualifies as the rarest. None of the bounty weapons would qualify either.what is the rarest Exotic Weapon?
IMO, Hard Light is probably the rarest. The rarest I ever was killed by in Crucible and don't know any body who has one.
2nd is Red Death, same as above but I have been killed by it more often than Hard Light but don't know anyone who has one
I'd say the rarest is probably a toss-up between the three Xûr hasn't sold: Universal Remote, Hard Light and Thunderlord. Hard to tell where the Vex Mythoclast falls on that scale since it's a rare drop but you can force it to drop with enough perseverance, unlike other Exotics.
If we want to be pedantic, the rarest is The Fate of All Fools, an Exotic Scout Rifle which only one Guardian has as far as I'm aware.