Its hard to say but if we are just talking about exotics then it would have to be the ones xur has never sold for the most part. But there is so little loot and the game has been going on for months I doubt anything is actually rare. I assume people covet the red death so much these days because when the dark below came out everyone wanted an exotic shard haha. So I suppose it would have to be the PS exclusives Monte and hawkmoon (neither of which I have), thunderlord, hard light (just got it last night), and MIDA(maybe). Can't remember if xur sold that last one.