I was reading a article that was posted on DiabloIncGamers website and felt like I would share it with the rest of the gaffers who are in expectations of patch 2.1 on PS4/Xbone
http://diablo.incgamers.com/blog/comments/diablo-3-legendary-drop-tables-updated-to-patch-2-1
This basically details drops rates and how certain legendary items are ranked as "common" or "uncommon" in their drops in dropping compared to other legendary items of that category. For example a Thunderfury would be a "uncommon" while Shard of Hate is "rare". If you read the full article you will get a good understanding at what your probability is in getting the item you need for your build and the amount of time you may need to invest to acquire it.
Reading this article does confirm one thing that amulets are by far (a good amulet) the rarest and most difficult to acquire.