I'm fairly inexperienced on Civ4, I typically play on Warlord and usually go for Space Race victories with one or two civs conquered, I've won on Noble but I'm trying to refine my skills by sticking with Warlord.
That being said... can someone please comment on the best economic system to use? Most of the time, I find myself using Mercantilism for the free specialist and keep it for quite a while. After that, what are the main benefits of going to Free Market or State Property? If my economy is already thriving, am I better served sticking with a free specialist rather than going to State Property? And does anyone ever use Environmentalism? I haven't found a use for it yet.
Really, the best economic system may change from one game to another. If you have a lot of land far away or on other continents State Property may be a good choice. It reduces the maintenance charges of cities and is very good for large, far flung empires. It'd work especially well on Archipelago. I usually use Free Market or State Property instead of Mercantalism, especially after Astronomy. The income from trade routes in each city and the lower maintenance may make either one a better choice.
Another big benefit from State Property is Watermills and Workshops. These become very powerful improvements in the later game under State Property. You can get some awesome hammer cities from State Property.
Another concern is Corps. You can't use them under State Property but if you use a lot of corps Free Market is usually the best Civic.
Environmentalism is more of a niche civic. I always run it if I have an OCC to get the extra gold from Forest Preserves and Windmills. If you have a lot of Windmills and/or Health problems it can be a good civic. For most games, however, you can trade or capture the resources you need and Free Market will usually be better.
I run Environmentalism a lot because I like OCC's. Finishes a game in about 2 hours.