First, to offer a complete survey of oolong tea would be impossible. There is too much variety and complexity within the category. It is best to get a general grasp of what oolong tea is, what differentiates mainland-Chinese oolongs from Taiwanese (or "Formosa" oolongs, as they are often called) and why oolongs from these two countries are inarguably the best.
Generally speaking, oolong teas are characterized by a level of oxidation somewhere between green tea and black tea. There is much variation in how oxidized any particular oolong is. Oolongs from mainland China are generally of lower oxidation. Sometimes called "pouchongs," these range between 20-40% oxidation, and have a light green, jadelike liquor and light, floral notes and hints of citrus fruits. In recent years, Taiwan has also taken to producing some of these, and some of their offerings are quite good. However, Taiwanese oolongs traditionally are more heavily oxidized, ranging from 40-80% oxidation, with a darker liquor sometimes approaching an amber or burnt sienna colour and darker flavor notes suggesting caramel.
Many of the most famous oolong teas come from Fujian province in China, a region that has a long tradition of producing them. Fujian Tie Guan Yin (pictured)

Perhaps the most bizarre and interesting Taiwanese oolong, however, is "Oriental Beauty" or Dong Fang Mei Ren. Produced only in Taiwan (although often the subject of ersatz imitations from elsewhere), Oriental Beauty is a variety of oolong whose unique flavor is influenced by an insect that chews on the leaves. The digestive secretions of this insect, known as a Cicadellidae or, colloquially, a leafhopper or tea jassid, seep into the leaves and change them on a chemical level. The resulting flavor is too complex to describe, with herbal, fruity, woody and gamy notes all contending for supremacy. The un-infused leaf is also very interesting: variegated and sort of shriveled, folded upon itself. Unfortunately, this is also very expensive stuff, usually going for about $30/100g.
If you're just venturing out into the world of oolong tea, a Tie Guan Yin from mainland China is a good starting point, as it features medium oxidation, a pleasantly inoffensive woody, "twiggy" taste and mild light orange color, and can usually be had for a reasonable price.
Finally, referring back to a question posed early on, why are the oolongs from mainland China and Taiwan the world's best? The answer, at least for now, is: no one else really makes them! While there are some oolongs produced in Nepal that are quite good (try some from the Jun Chiyabari estate, for example) and some made elsewhere in the world (Vietnam and India make them as well) no other nations produce enough to be serious contenders, and what they do produce tends to fall short of the high quality of Chinese and Taiwanese oolong.
Our next article will discuss one of the world's best oolongs, the roasted Taiwanese Tie Guan Yin. While expensive, this tea is an unparalleled experience and a must for anyone interested in the best the style has to offer.