Mine is Black Hammer. I have never gotten it or seen it drop in any of my runs. Also it's the only Crota weapon I don't have.
Edit 1: For all of those people who are saying Thunderlord, just a few hours ago I got it in IB.
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Crackhammer, Gjallarhorn, Praetorian Foil, Praedyth's Timepiece, a B-Line Trauma without Mulligan (I've had about four drop with it), and an MG18A with Field Scout (3 or 4 without it). I also don't have the 4th Horseman, NecroCrux, or Swordbreaker.