Serfdom is extremely limited, but I can see running it at the start of a Golden Age (when you do all of your normal civic switching), since you want to grow into a GA and keep on growing population during it, get 7-11 good turns with it, then switching back to your default Slavery/ Caste System right before the GA ends. Serfdom is probably tied with Caste System as the best labor civic during GAs.
Police State is vital if you are going for domination during the later eras. War Weariness increases fast and you don't want to deal with both WW AND Emancipation frownies. The production bonus is great too.
For my personal play style, I tend to leave the religious civics well enough alone. I don't find any of the religious civics pre- Free Religion to be worth a turn of Anarchy switching to, AND having to spend another turn in Anarchy to adopt a religion (causing diplomatic problems), AND having to build missionaries to spread religion to the relevant cities. Free Religion comes early enough in the game that I don't feel like I'm missing anything out of that entire tree. That being said, if I had a different play style or was running a Spiritual leader, it would be different. Organized Religion is a must for a wonder-spam strategy and useful if you acquire a new religion to spread post- scientific method, and Pacifism for SE/ Golden Ages like Serfdom.
I find Representation to be worthless if you don't get it from the Pyramids. In normal situations by the time you get Constitution your Tech cities already have towns, and the Happiness is only useful early. Representation for me is only for One City Challenge.
For me the Economic civics are the most lop-sided. First, they all become available really late and all at once, practically. Second, Free Market and State Property are just way, way, way ahead Mercantilism and Environmentalism. Environmentalism used to be peachy in OCC but its pointless now because of National Park.
So my least used are:
Environmentalism. Not even useful to diplomatic purposes.
Mercantilism. Only use is if you are desperate to get on Toku's good side.
Theocracy. Only worth it to get on Isabella's good side.
Representation. Useless by the time you normally get it unless you have serious economic or happiness issues.
Cheers,
Dai