Magelo provides very useful info in this regard, with exact frequency of drops experienced by type of mob. Of course, it can only access data from players who are willing to contribute but it's useful samples. I used it often when chasing collectibles that drop from mobs and I found it usually on the mark.
Adjectives based on anecdotal evidence can always be misleading. We've all been there pharming a drop and thinking it's been 100 kills without seeing it and when we check our logs it was more like 19. Estimated actual drop experience might be more useful than descriptive terms, so say maybe "Drop rate ~1/100" rather than "Rare" since "Rare" can ,mean different things to different players. Just a suggestion...
Edited, Jan 8th 2017 5:38am by Sippin
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Sippin 115 DRU **** Firiona Vie ****Agnarr
FV: 115 WAR ENC CLE MAG WIZ SHD SHM Master Alchemist ROG Master Tinkerer & Poison-Maker
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Agnarr: 65 DRU ENC SHD MAG CLE ROG WIZ BRD WAR