I don't know, probably Necrochasm, but I haven't actually seen numbers of who has what. The one that feels the rarest to me is the Dreamwaker I blew over 15,000 glimmer on. Void, tracking, tripod. I am currently saving up glimmer to try rerolling my Admonisher III some more. Trying to get arc, tracking, tripod. I'm about 10,000 glimmer in so far and broke. Is it worth it ... NO, but I want to have a legendary tracking tripod launcher in each element. HoC technically doesn't have tripod, but close enough.