so whats the conclusion?
im confused..
what should i use in what situation?
Cottage spam never hurts. Build enough farms to hit pop 20, then build enough windmills to hit pop 20, then build cottages on every other tile, even grassy hills. Then, if you have anything left over, put a mine on it. If you can't put a mine on it, put a watermill on it.
Some people prefer to specialize their cities, but that's not necessary. Simply switch to Universal Sufferage, Free Speech, Free Religion, Emancipation and Free Market. You'll probably hit #1 in production, GNP, and trade. This route lends itself to either Space Race or Cultural. You can also do Diplomatic, since you'll be the first to Mass Media, and thus easily build the U.N. You can still get Conquest or Domination, of course. Without ever building a single workshop in your entire empire, you should
still be able to pump out tanks, gunships, and marines in just 1-3 turns each, from any city in your empire.
If you want to specialize your cities, you might choose to put windmills and cottages around your commerce cities, then build craploads of workshops and watermills around all the others. This only really works in State Property. Universal Sufferage and Free Speech become much less attractive, so maybe you want to stick with Hereditary Rule or Police State. Most people go with Slavery, Bureauracry and Theocracy, under this model. It's great for warmongers who win through domination or conquest. There isn't much call for anyone to have this much production, unless they're using Epic or Marathon speed. These speeds make the game much, much easier to win, effectively reducing the difficulty level by one (epic) or two (marathon).
Maybe you want a specialist economy. In that case, you pretty much want to farm everything and make as many scientists as possible. Obviously, this only works under Representation, and most people build the Pyramids in order to get access to Representation early. Or you could declare war on whoever built it, as their defenses will probably be a bit poor. Mercantilism, Caste System, Bureauracy and Pacifism are quite popular under this model.
Enlightened leaders often prefer to eschew the "distasteful" civics, such as Slavery or Police State. They will often choose Representation, Bureaucracy, Emancipation, Environmentalism, and Free Religion. Their cities will sport large amounts of lumbermills (for health and happiness), ample farms (for specialists), and a good amount of mines (for production).
Your choice of civics affects which improvements you build, and vice versa. If you want to run Slavery, make sure you have enough food to continually grow back those people you whip. If you want to run Universal Sufferage, then don't whip away your population. You need those cottages worked.
Your traits also affect which improvements you build. If you're financial, you probably want to make sure you're building windmills on coastal hills. Watermills might also be somewhat useful, too, even if you're not under State Property. Workshops might not be as attractive, seeing as how you could be building cottages on those tiles. Also, you end up building more coastal cities, seeing as how you get three commerce from coastal tiles.
Sometimes, I let a city languish at size 7, if it's just a resource grab. There's no reason to grow that city to size 15, if I'm only going to be working tundra, ocean, or non-fresh farms. This is also often a problem if the city is surrounded by desert, ice, or peaks.
Obviously, if you're playing marathon, deity games, your strategy will be different than if you're playing quick, noble games. I find marathon to be a crutch, so I usually stick to normal or epic. I usually stick to the middle difficulty levels, such as Prince or Monarch. Monarch is a real pain with the Better AI, though!