Protestant is the best. It's more adaptable than Hindu, which used to be the clear winner
Clear winner? Haha no. Hindu isn't even top 5. Catholic, Reformed, Protestant, Sunni, Shia, Ibadi, Orthodox, and Coptic are all better.
Why are they better? They have more triggered modifiers for missionaries and access to DotF, to go with numerous powerful decisions or piety + temporary decisions. Even today, Shia can trivially pick up 20% morale with piety + DotF. Hindu can, if it sacrifices other advantages, get 5% discipline by comparison while converting much more slowly. Hindu only wins out on nations that can get more +heathen tolerance so they don't have to convert.
Most pagan faiths are also a joke for that reason. When you have intolerance, every province in the wrong religion gets penalties in addition to handing you more rebels, and these faiths can't possibly keep up converting with conquest rate unless you're slow.
Thinking on it, a Hindu republic probably provides the widest and most flexible array of bonuses (also, one of the only religions to reduce coring costs).
And yet a Catholic one can just buy stability and diprep using PI and outstrip the bonuses even if it doesn't control the Curia a single day.
Edit: Though -1 diprep is painful, Buddhism was pulled out of the trash can with the flip to Karma penalties and access to 4 missionaries with religious + 2 triggered modifier provinces (it is the only non-Abrahamic that gets 2). I no longer rate it as true trash tier like it was in 1.13, it's much more middle-of-pack now.
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