Darkest Hour Full itself is already pretty good in many ways and absolutely worth, if not even recommended, to play as a standalone if you start with DH.
I think the question which mod is best is a difficult one because:
1) I am not sure how many players there really are who have played all the mods in a way that afterwards they have a base for saying THIS one is the best. Personally I surely haven't such a base, there are still quite many mods I haven't even touched yet.
2) Many mods have many different targets what they wanted to improve: some are purely graphical or about music, some country specific, some to improve "realism" while others strive for the exactly opposite to offer alternate history, even fantasy settings or incorporate beloved worlds and settings from books, some concentrated on better AI, some on raising difficulty. The biggest ones are those who tried to bring together many modders' work to one supermod (which often enough comes with also vast problems).
Some examples:
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World in Flames II (WiF2) was made with the intention to give a player a much harder challenge especially when playing Germany. So initially it was definetly not meant for the beginner but after you got bored beating the computer players easily and looking for something rather crazily difficult.
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Edge of Darkness (EoD) was advertised as "trying to increase historical accuracy as well as balancing" and gave much more spice to the usual game in forms of many more events.
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Kaiserreich calls itself "alternate history mod"
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EODAIP - an AI focused Darkest Hour Mod
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Nick's WW2 Multiplayer Realism Mod has given itself 2 big aims: multi-player on the one hand and then absolutely and vastly overworking whole aspects of resources, distances, ships, logistics... and and and to make them more "realistic" in a way which is probably unique.
I am playing DH on and off since years now. When looking back, I must say, my greatest inspiration for "What mod do I want to play" came all too often from starting to read an
AAR (After Action Report) and then reading them with more and more pleasure.
And I think this would be my recommendation:
# Play "DH full" to learn the game.
# Read some AAR... and get inspired.
# When having "something" in mind you want to play, a specific country, a specific setting, perhaps plus some specific wishes (should have many and as historically accurate events... OR give me most freedom to play...), come back and ask those specific questions and what mod would be best for this.
edit: typos and adding a few links