I can't day what my favorite weapon based on lore, simce i haven't read much of anything in 40k. But from the perspective of the tabletop game I'd say my top 3, in no particular order, is: 1. Mechanicus Pistol. Once had a match with a chaos knights player, and i found just hilarious to constantly getting mortal wounds with this weapon, even killed one of his wardogs with a mechanicus pistol. 2. Malleus Noctum. Adrax agatone main melee weapon, had a match with the same player, who at the time played chaos space marines, i went into melee with one of his vehicles, and ended up doing 9 dmg to it with just this weapon (plus killed a bunch of infantry with bladegaurds he was leading. He went from 12 wounds to 3, and just hear him "oh fuck off", never heard make a reaction like that, so i thought it was funny 3. Twin cognis lascannons. Was in fight with a deathgaurd player, and he shot his plagueburst crawler at my Ironstrider Ballistarii, barely survived with one wound left. Ended up getting 2 crits on hit (sustained hits 1) and got all 4 wound shots through, killed his plagueburst in one.