1) You need to find the recipes first. I highly doubt that you have found most of them already, I have only found one so far (except for the 100% Malthael drop of course).
2) There is no general "pick this", "pick that". If they would all be useless except for one, why would there be multiple? They all serve a purpose. Pick the one that looks most appealing to you. It depends on your class, spec, playstyle, favorite spells, current gear, ...
When you find a plan, check out if the items that you can craft from the plan look interesting. If so, farm the material. That's the way to go about it.
Most likely higher drop chance on higher difficulty, but no one knows. Diablo 3 has never been about exact percentages (except for ubers), they want us to keep in the dark in terms of an item's actual drop chance.
I try all day to huntEssence of the Twin Seas, but i dont seeRazormouth spawn inBlack Canyon Mines ( form T1 to T6). The chance of finding Unique is too low How to collect Leg Material Human Cartilageis the Leg easiest i think, becasue we always find theManglemaw in game, i collect 5Human Cartilageafter 6 hour kill him LoL Someone can tell me ho to findEssence of the Twin Seas,. I cant findRazormouth spawn inBlack Canyon Mines !
I just want to provide an update for the location of Shimmering Quill (Unbound Bolt mat). It drops from Sarkoth, which spawns in the dank cellar (not dark) on Old Tristram Road. The spawn rate of this mob is approximately 10-20% from my experience.
Not sure if this was posted already but I received a Shimmering Quill from a Quilback in Act 1. I believe it was the Quillback that takes all the treasure from the chest in the basement right before the path that leads directly to Old Tristam.