If you can handle the happiness, Caste is hard to beat - it's like playing the whole game in a Golden Age, with food too!
There's very little reason not to run Free Church, if only for the building bonus. RoM has lots of very good buildings (relative to Vanilla/other mods). Only diplomatic considerations should change this. +20% research? Build a library!
Agreed that Communism/Fascism are too good - needs to be another modern civic in that category to compete. I beeline Monarchy early for roads (+1 from mines) and to turn off the city maintenance penalty, but Republic is usually better once its available. Democracy is a disappointment.
Federalism's lower maintenance costs can make it worth doing if you have extensive colonies/overseas conquests, while Senate's trade route food bonus (combined with the one from Mercantilism) along with the Mil production bonus (with the more expensive Industrial units) also makes it playable once you hit IPorts.
Private (happiness, science, health - what's not to like?) is worth beelining, but Corporate is key in the long game as the Corp buildings are strong, but the upkeep is expensive. In the late game, I'll build them all in each city by switching corporations.