I would like to know who among the deities of the Forgotten Realms has the most followers.
I'm planning to create an Order on my campaign that unites followers from the greatest gods that works as some sort of inquisition against least popular gods/evil cults.
The number of worshippers is not necessarily countable: the polytheism of the Realms is such that people venerate many members of the pantheon. You might have a patron deity, but you might still pray to another on particular occasions. Quoting from the 2e sourcebook Faiths & Avatars (F&A):
What matters to a particular Realms power is not that a follower worships someone else - most everyone in the Realms worships several someone elses - but rather which other powers are venerated and which are appeased, and how serious a person's offerings and worship are to other deities.
Hence an inquisition like the one you describe in your question does not sound very likely on Faerûn.
Moreover, many powers are allied. For example, Mystra has Velsharoon, Azuth and Savras serving her. Deneir, Gond, Lliira and Milil serve Oghma, etc. Hence an alliance of greater gods against the lesser ones again seems unlikely. Actually, F&A also provides a rough answer to the following question: Given that the power of the deities come from their followers, why do the greater gods let the lesser ones live? (1) Greater powers protect them against other greater powers; (2) Weaker deities pay some amount of divine power to the greater deities, willingly or through extortion. So again, it is unlikely that the greater deities would gang up against the lesser ones.
But if we leave aside the motivation for your question, and focus on the power of each deity from a pure mechanical point of view (as power correlates strongly with the collective amount of worship received), there is one source that provides the information you are asking for: 3e sourcebook Faiths & Pantheons (F&P). Beyond the demi-lesser-intermediate-greater categorisation that appears in most editions of D&D, this book also defines a "divine rank, which is similar to a character's level". While 4e has changed a lot of things about the FR pantheon, 5e appears to be a very strong return to the flavor of 3e FR, with dead or missing deities returning to pretty much to how they were in 3e (or even 2e). So the divine rank from F&P should give you an idea.
According to F&P, a divine rank of 21+ is only for an overdeity (like Ao), and such deities are not interested in mortal affairs. So the "big" guys are the greater powers, with divine ranks in the range 16–20. Quoting F&P, greater deities "have millions of mortal worshipers", whereas the intermediate ones "have hundreds of thousands". (I assume this is again not an ordinary count, but a collective effective measure.) I list below the divine ranks of greater deities from F&P:
Rank 20: —
Rank 19: Chauntea
Rank 18: Mystra, Shar, Silvanus, Tyr
Rank 17: Bane, Cyric, Kossuth, Lathander, Tempus
Rank 16: Kelemvor, Oghma, Sune, Talos
Note: The divine rank for a few greater deities is not listed: Akadi, Grumbar, Istishia, and Ubtao. The first three of these can be considered similar to (or weaker than) Kossuth, as they are all elemental powers. Ubtao's faith is strong only around Chult.
Reading through the deity descriptions in Sword Coast Adventurer's Guide, the power level differences appear to be less pronounced compared to earlier editions. But one can perceive similar power structures amongst the deities. For example, Deneir and Milil are said to be "sometimes thought of as being in service to Oghma". So you might perhaps want to slightly modify the list above, but overall I hope the list provides enough insight to finding the answer to your question.
I have read a ton of the Forgotten Realms books over the years starting when I played D&D 2.0. Now the Gods have changed a fair amount since then. I generally follow the Wikipedia source here:
https://en.wikipedia.org/wiki/List_of_Forgotten_Realms_deities
It was noted in the material used to play D&D version 2.0 that greater gods had the largest following and tend to be of the major areas Ex. war, love, nobility, paladins, etc
With that said the most powerful of the gods are the greater gods listed in the above link. When I create a world I often use these to help players as making my own is very time consuming and many of us are familiar with the books and playing in the forgotten realms when we played version 2.0.
With that said I suggest you cover the basics that the Gods would represent. Examples would be:
War Death Tyranny/betrayal/hatred Love/Life Nature/farming All the elements Chaos Magic Knowledge/Lore/secrets
Hope this helps
The official complete pantheon of the Forgotten Realms in 5e is in PHB p.293 (online listing here). There is no distinction between greater and lesser gods, so I presume the line of distinction will be on account of popularity.
Unfortunately, there is no (and probably will never be) a complete demographics chart by race of the FR setting. The only thing that may exist is the racial breakdown of specific cities and/or maybe countries (e.g. Waterdeep).
To answer your question, I'm going to make some assumptions:
My proposal:
However, you could probably go with another pantheon, where the lines are more clearly drawn, for example Dragonlance...
