Olleus
Deity
I'm not one of those people who is overly obsessed with Civ IV (it does some things well but I also prefer aspects of the later games), but I do think that is still the best implementation of religion in that a) you can choose your state religion, and b) the importance of this choice is primarily diplomatic, it's not about increasing yields or some kind of religious contest.
Well it was a contest in the sense that each city could only ever have 1 religion. And the religion branch of civics did convert religion into substantial yields (ie, +25% production for cities with the state religion) in a non-diplomatic way. Beyond that, the gold generated at the Holy City per city that followed the religion was huge! It could easily overcome every maintenance cost and let you run at 100% science for ever. There's a reason Great Prophets were called Great Profits lol. I think Civ5 improved things by having pantheons and then choosing beliefs which gave religions more character. It was the missionary spam and religious combat that went in the wrong direction IMO.