How many soils? There are far too many to try to list, but probably hundreds of thousands. Soil scientists understand that soil forms from several 5 interacting forces that include Climate, Organisms (plants and animals), Relief (shape of the landscape and position on it), Parent materials (the stuff in which soil forms), and Time (how long the soil has had to form). We call these ClORPT for short, but since there are so many combinations of those factors throughout the world, there are so many different soils.
Soils that formed in tall-grass prairies are some of the most fertile soils on the planet. They are used widely to grow corn, soybeans, and many other food crops.
Most fascinating – that is an interesting question. Almost every soil I examine fascinates me to a degree, especially when it is the first one of its type I have seen. Soil scientists in the USA use a classification system with 12 orders (think of this like the Divisions in the botanical classification system). There are several other classification systems that have been developed, but though the terminology varies, the factors used to put the soils into groups share some commonalities. You can see some different soils at this site, http://www.cals.uidaho.edu/soilorders/orders.htm
You can learn about one of the soils in your state of the hundreds to thousands that has been selected as especially important, check out your state soil. http://www.nrcs.usda.gov/wps/portal/nrcs/detail/soils/edu/?cid=stelprdb1236841 And check this site for more information on that soil. http://www.soils4teachers.org/state-soils This list is growing as we get them done, so if you do not find your state listed yet, do not give up, it will be.