Transmogrification / Collections addon, "ALL THE THINGS" on Twitch. (Curse Client)
https://mods.curse.com/addons/wow/267285-all-the-things
The Primary Collections Window can be moved, resized, and contains every item known to the prebuilt Item Database module (ItemDB). Since it uses a predefined database, it computes what you're missing very very quickly. A lot faster than I've seen done in other addons of this nature.
You can even right click on groups of items, instances, encounters, difficulties, and tier tokens to pop them out into their own standalone windows:
The addon also shows you where you can get any collectable item, whether or not you have collected it, and any shared appearances. (all of these settings can be turned off)
Contributors:
If you would like to contribute to the addon, there is now a Discord server: https://discord.gg/9GFDsgy
Please report any missing items! Any and all suggestions are welcome!