I've used most of the above and just about all of them have their place. I have gone to using black nitride most often for two reasons:
-it adds little to no dimension to the components so it does not alter barrel timing or bedding.
-it protects the inside of the bore
⬆️ what he said, in spades. The nitride on CZ rifles is both attractive and tough. Same goes for the version on S&W pistols, which is over SS for a double whammy against bad things. Since it goes into the metal, even if it starts to wear (which hasn’t happened on any of mine yet), the protection and lubricity remain, or so “they” say.
I wish more makers would go to it instead of the crappy black bluing they use and save us the expense and trouble.