A reddit user named mr_Irony, who’s ironically a pretty nice guy for putting together the following list, has drawn up a list of Bounties he noticed to be highly efficient at providing Legendary crafting materials in Diablo 3: Reaper of Souls’s Adventure Mode.
"I noticed the best way to get most of them is to get a bounty that guarantees an interaction with the rare that drops the item,” he wrote.
For those not in the know, Adventure Mode allows players to skip past all of the game’s story and undertake a wide variety of tasks throughout the world of Sanctuary, killing monsters and completing specific quest objectives. These quests bring players into close contact with unique named rare monsters (the monsters with purple-colored names), many of which have the potential to drop Legendary crafting materials.
So if you’re in the need for Legendary crafting materials and want to build specific items, this list should serve as a guide on which Bounties to undertake.