Green tea has been shown to have the highest content of polyphenols, which are chemicals with potent antioxidant properties (believed to be greater than even vitamin C). They give tea its bitter flavor. Because green tea leaves are young and have not been oxidized, green tea has up to 40 percent polyphenols, while black tea only contains about 10 percent.
Another interesting note: Green tea has a third the caffeine of black tea. Even better, it's been shown to yield the same level of excitement and attentiveness, but in more even levels than the ups and downs associated with other caffeinated drinks. Just don't drink milk with it; the casein in milk has been shown to inhibit the beneficial effects of tea.